This knowledge shared by both systems is modeled as being a "Transfer Syntax".

The specification of a Transfer Syntax includes the exhaustive definition of all the values that the applications can exchange, as well as the detailed definition of the octet streams (Representations) attached to these values, which are used in the exchange over the session layer.

Nota Bene: This notion of Transfer Syntax can be generalized to include the cases where the session layer is not used, but another "pipe" of some sort.