Contract interfaces define an API between smart contracts and software (wallets, exchanges), allowing semantic-aware user interaction with arbitrary contracts.

LNP/BP Standards Association maintains standards and the registry for most-common used interfaces, named RGB-20, 21, 22 etc., however for independent vendors it is optional whether they'd like to standardize custom interfaces. Standardization of interfaces doesn't affect wallet compatibility; RGB standard libraries provide a way to work with any interfaces, even non-standard ones.