The OSI model uses a Service Access Point (SAP) to define the communication between layers (like Network, Transport, Session, and the other layers of the seven-layer model), that is to identify which protocol should process an incoming message.
Within a given layer, programs can exchange data by a mutually agreed-upon protocol mechanism.
The most common reference to SAP, including a Source Service Access Point (SSAP) and a Destination Service Access Point (DSAP) refers to the boundary between the Data Link Layer and the Network Layer.
It is common to think of SAP only in terms of its use at Layer 2, specifically in its Logical Link Control (LLC) sub-layer as defined in the IEEE 802.2 standards.
Standard Network layer protocols have been assigned reserved LLC addresses, as recorded in ISO/IEC TR 11802-1.
One way is by local assignment of LSAPs, for which the other half of the LLC address space is available.
On other network types, the LLC and SNAP headers are required in order to multiplex different protocols on the link layer, as the MAC layer doesn't itself have an EtherType field, so there's no alternative framing that would have a larger available payload.