They are. BT-157 a BT-158 mandatory in Slovak e-invoice?
Short answer
BT-157 a BT-158 are scheme identifiers in classificationcii items. They are practically dealt with when the invoice contains a classification indication of an item, such as a code from a dial. If the classification of the item is not used, a fictitious code should not be created to fill in the scheme. If you use it, you should also indicate the scheme by which the value is read.
Optional context
First, see if you're sending a classification.
Meaning of value
Specifies the dial or system from which the value comes.
Mapping
Internal codes need to be converted to supported entry.
Conditional rules
The obligation depends on the data group used.
Conclusion
BT-157 a BT-158 are important when using the classification of items; their aim is to explain the code scheme, not to force companies to create artificial item classifications.
Financial Administration of the Slovak Republic, FAQ 9/VAT/2025/IM k e-invoice, July 2026 update
Not legal advice. Machine-translated answer, not yet manually reviewed. For the exact wording, see the Slovak original.