Before two LUs can communicate, VTAM® must establish a session between them. In any session between logical units (LUs), one LU, the primary logical unit (PLU), acts as the primary end of the session (also called the primary half-session); and the other LU, the secondary logical unit (SLU), acts as the secondary end of the session (also called the secondary half-session). For some LU types, the PLU has more control over communications than does the SLU. Sessions shows an example of the SNA session-initiation procedure.

The type of macroinstruction issued by the initiator of a session determines which LU is to be the PLU and which LU is to be the SLU in that session. If the application program issues a REQSESS macroinstruction to initiate the session, that application program becomes the SLU in the established session. If an application program issues a SIMLOGON macroinstruction, it becomes the PLU in the session. The PLU actually establishes the session by sending a BIND request to the SLU.

The system services control point (SSCP) assists the LUs in establishing and terminating sessions. Although only one SSCP (a single-domain network) is discussed in the following sections, the session-establishment and termination procedures involved for a multiple-domain network are very similar, with the SSCPs cooperating to give the appearance of a single SSCP to the LUs involved.