Internally Firebird/InterBase® has several possible “engines”, and when attaching to a database it must decide which engine it is to use. The logic for deciding which engine to use is called the y-valve. (Steve Tendon's definition.) Among other things, the y-valve should decide whether to use a direct access to the database (local database) or to connect as a client to a remote Firebird/InterBase® server and what server version to use for reading a given ODS (for the cases where the same Firebird/InterBase® is able to read different ODS versions).