SK-CIUS (Slovak Core Invoice Usage Specification) is the Slovak national extension of the EN 16931 European e-invoicing standard. It defines additional mandatory requirements specific to Slovak VAT law and the SAPI-SK PEPPOL infrastructure, on top of what EN 16931 and PEPPOL BIS Billing 3.0 require.

Which fields are mandatory in a Slovak e-invoice?

Mandatory fields include: Invoice ID (BT-1), Issue Date (BT-2), Invoice Type Code (BT-3), Currency (BT-5), Seller name (BT-27), Seller address (BG-5), Seller VAT ID (BT-31 or BT-32), Buyer name (BT-44), Buyer address (BG-8), at least one invoice line with description and net amount, tax category, and all total amounts (BT-106 through BT-115). Missing any of these triggers a fatal validation error.

Is a bank account number mandatory on a Slovak e-invoice?

Conditionally mandatory: the payment means (BG-16) including bank account (BT-84/BT-86 for IBAN/BIC) is required when the invoice is payable by bank transfer. If payment is by other means or the invoice is a credit note with no payment due, it can be omitted.

Standard · SK-CIUS · BT / BG Fields

SK-CIUS — Mandatory, conditional and optional fields

Which fields must appear in every Slovak e-invoice, which depend on context, and which are optional. Based on EN 16931, PEPPOL BIS Billing 3.0, and the Slovak national extension SK-CIUS.

📋 Technical reference ⏱ 8 min read 🔄 June 2026 🇬🇧 English

Standard architecture — three layers

Slovak e-invoicing is built on three stacked specifications. Each layer adds requirements on top of the layer below:

EN 16931

European semantic standard — defines what information an invoice must carry. Sets the baseline for all EU member states.

PEPPOL BIS 3.0

Network delivery profile — defines UBL 2.1 XML structure, CustomizationID, ProfileID, and network-specific rules (R-series rules).

SK-CIUS

Slovak national extension — additional rules from Slovak VAT Act No. 222/2004. Makes certain optional EN 16931 fields mandatory for Slovakia. Rules prefixed SK-CIUS-xx.

In practice: a valid Slovak e-invoice must satisfy all three layers. The validator at /peppol-validator/ checks all of them in a single pass.

Field reference by category

MANDATORY Fatal error if missing — invoice rejected

CONDITIONAL Required in specific circumstances

OPTIONAL Recommended but not required

SK Additional requirement from SK-CIUS

BT/BG	Field name	Status	Notes
Invoice header
BT-1	Invoice number	MANDATORY	Unique identifier assigned by seller. Free format.
BT-2	Issue date	MANDATORY	Format YYYY-MM-DD.
BT-3	Invoice type code	MANDATORY	380 = Invoice, 381 = Credit Note, 384 = Corrected invoice.
BT-5	Invoice currency	MANDATORY	ISO 4217 code, e.g. EUR. All amounts in this currency.
BT-6	VAT accounting currency	CONDITIONAL	Required if different from BT-5 (foreign currency invoices with EUR VAT).
BT-9	Payment due date	CONDITIONAL	Required when PayableAmount > 0 and payment terms apply.
BT-10	Buyer reference	CONDITIONAL	Required if buyer provides a reference (e.g. purchase order number). Pass through whatever the buyer sends.
BT-13	Purchase order reference	OPTIONAL	Buyer's order number. Recommended for B2B matching.
BT-25	Preceding invoice reference	CONDITIONAL	Mandatory on Credit Notes (type 381). References original invoice number.
BT-26	Preceding invoice issue date	CONDITIONAL	Mandatory on Credit Notes together with BT-25.
Seller (Supplier) — BG-4
BT-27	Seller name	MANDATORY	Full legal name of the issuing company.
BT-33	Seller legal form	CONDITIONAL SK	SK-CIUS: required for companies registered in ORSR (Business Register). Typically "s.r.o.", "a.s." etc. Maps to `CompanyLegalForm` element.
BT-30	Seller legal registration ID	CONDITIONAL SK	IČO (Company Registration Number). SK-CIUS requires this for registered companies.
BT-31	Seller VAT ID	CONDITIONAL	Required when seller is VAT-registered. Format: SK + 10 digits (e.g. SK1234567890).
BT-32	Seller tax registration ID	CONDITIONAL	DIČ (Slovak tax ID). Required when seller is not VAT-registered but has a tax ID.
BG-5	Seller postal address	MANDATORY	At minimum: street, city, postal code, country code (SK).
Buyer (Customer) — BG-7
BT-44	Buyer name	MANDATORY	Full legal name of the recipient.
BT-46	Buyer VAT ID	CONDITIONAL	Required when buyer is VAT-registered. Used for PEPPOL routing and VAT verification.
BT-47	Buyer tax registration ID	CONDITIONAL	DIČ of the buyer. Required when BT-46 is not present.
BG-8	Buyer postal address	MANDATORY	Street, city, postal code, country code. At minimum country code is required.
Delivery — BG-13
BT-72	Actual delivery date	CONDITIONAL	Required if different from invoice date. Relevant for VAT point-in-time determination.
BT-73	Delivery period start	OPTIONAL	For invoices covering a period (e.g. monthly services).
BT-74	Delivery period end	OPTIONAL	Used with BT-73 for period invoicing.
Payment — BG-16
BT-81	Payment means type code	CONDITIONAL	Required if payment information is included. 30 = credit transfer, 58 = SEPA credit transfer.
BT-84	Payment account IBAN	CONDITIONAL	Required for bank transfer payments. Slovak IBAN: SK + 22 digits.
BT-86	Payment service provider BIC	OPTIONAL	BIC/SWIFT code of the bank. Recommended for cross-border payments.
BT-83	Remittance information	OPTIONAL	Variable symbol (VS) or specific symbol for Slovak bank payments. Strongly recommended.
VAT — BG-23 (Tax subtotal)
BT-116	VAT category taxable amount	MANDATORY	Net amount for each VAT rate applied.
BT-117	VAT category tax amount	MANDATORY	VAT amount for each rate. Must carry `currencyID` attribute.
BT-118	VAT category code	MANDATORY	S = standard (20%), R = reduced (10%), Z = zero, E = exempt, O = outside scope.
BT-119	VAT rate	CONDITIONAL	Required for S and R categories. Slovak rates: 20% standard, 10% reduced.
BT-120	VAT exemption reason text	CONDITIONAL	Required for E (exempt) and Z (zero-rated) — must reference the legal basis.
BT-121	VAT exemption reason code	CONDITIONAL	Codified reason for exemption. Use VATEX-EU codes.
Totals — BG-22
BT-106	Sum of line net amounts	MANDATORY	Sum of all BT-131 values.
BT-109	Invoice total net amount (excl. VAT)	MANDATORY	After allowances/charges. Must equal BT-106 ± document-level adjustments.
BT-110	Invoice total VAT amount	MANDATORY	Sum of all BT-117 values.
BT-112	Invoice total amount incl. VAT	MANDATORY	BT-109 + BT-110.
BT-115	Amount due for payment	MANDATORY	BT-112 minus any prepaid amount.
Invoice line — BG-25
BT-126	Invoice line identifier	MANDATORY	Sequential line number (1, 2, 3…).
BT-153	Item name	MANDATORY	Description of the goods or service.
BT-129	Invoiced quantity	MANDATORY	Must include unit code attribute (e.g. `unitCode="C62"` for pieces).
BT-130	Invoiced quantity unit code	MANDATORY	UN/ECE Rec 20 unit codes. Common: C62 = piece, HUR = hour, KGM = kilogram.
BT-131	Invoice line net amount	MANDATORY	Quantity × unit price ± line-level allowances. Basis for VAT calculation.
BT-146	Item net price	MANDATORY	Price per unit excluding VAT. Part of BG-29 (Price details).
BT-151	Line VAT category code	MANDATORY	Must match one of the tax subtotals in BG-23.
BT-152	Line VAT rate	CONDITIONAL	Required for S and R categories on line level.

SK-CIUS specific rules

The SK-CIUS extension adds 12 validation rules on top of EN 16931 and PEPPOL BIS. These are specific to Slovak law and infrastructure:

Rule	Severity	What it checks
SK-CIUS-01	FATAL	Seller VAT ID (BT-31) must use format SK + 10 digits when seller is SK VAT-registered.
SK-CIUS-02	FATAL	Buyer VAT ID (BT-46) must use format SK + 10 digits when buyer is SK VAT-registered.
SK-CIUS-03	FATAL	IČO (BT-30) must be present for sellers registered in the Slovak Business Register (ORSR).
SK-CIUS-04	FATAL	BT-33 (CompanyLegalForm / legal form text) required for ORSR-registered sellers.
SK-CIUS-05	FATAL	IBAN (BT-84) is mandatory when payment means code is 30 or 58 (bank transfer).
SK-CIUS-06	FATAL	Invoice date (BT-2) must not be in the future by more than 30 days.
SK-CIUS-07	FATAL	TaxSubtotal/TaxAmount must carry `currencyID` attribute matching BT-5.
SK-CIUS-08	FATAL	LegalMonetaryTotal elements must appear in the specified order per UBL schema.
SK-CIUS-09	WARNING	Payment reference (variable symbol / BT-83) recommended for bank transfer invoices.
SK-CIUS-10	WARNING	Delivery date or period recommended when service delivery differs from invoice date.
SK-CIUS-11	FATAL	Credit Notes (type 381) must include BillingReference with original invoice ID (BT-25) and date (BT-26).
SK-CIUS-12	FATAL	TaxPointDate on Credit Notes must not precede original invoice date.

Slovak VAT rates (2026)

Slovak VAT rates applicable to e-invoices:

20% — standard rate (most goods and services)
10% — reduced rate (basic foodstuffs, medicines, books, accommodation, selected services)
5% — reduced rate (selected foodstuffs introduced 2024)
0% — intra-EU supplies, exports, specific exempt supplies

Use VAT category code S for both 20% and 10%/5% rates (reduced is still "standard taxed" in EN 16931 taxonomy). Use Z for zero-rated, E for exempt, O for outside scope (e.g. intra-EU supplies).

Common mistakes by foreign companies

Missing IČO (BT-30): Foreign sellers often omit the Slovak company registration number. If you have a Slovak IČO, it must appear.
Wrong VAT ID format: Must be SK prefix + 10 digits, no spaces or slashes. Not the same as the DIČ format.
Missing currencyID on TaxAmount: Triggers SK-CIUS-07. Every TaxAmount element needs currencyID="EUR".
Credit Note without BillingReference: Triggers both BR-55 (PEPPOL) and SK-CIUS-11. Always reference the original invoice.
Wrong CustomizationID: Must be exactly urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0 — no variations.

Check your invoice against all SK-CIUS rules

Upload your UBL XML — the validator checks EN 16931, PEPPOL BIS, and all 12 SK-CIUS rules in a single pass. Fatal errors and warnings shown with explanations.

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