An LU might have more than one session at the same time with another LU. Such sessions are called parallel sessions. LUs that can support parallel sessions are parallel-session-capable LUs. When the first session is established between LUs, part of the session establishment process defines whether the partners are capable of parallel sessions. If either LU is not parallel-session capable, the session is said to be a single session. If both are parallel-session capable, the session is called a parallel session, even if it is the only one in existence at the time.

Parallel and single sessions use different protocols. Establishing a session as a single session restricts the LUs to only one session at a time. LUs capable of only a single session can support only one transaction program at a time. LUs that are capable of parallel sessions can support more than one session at a time and more than one transaction program at a time. Figure 1 shows a comparison of type 6.2 LUs capable of only a single session and type 6.2 LUs that are parallel-session capable.

In parallel sessions, an LU can have more than one TP at a time, and a TP can have more than one session at a time.

LUs that are parallel-session-capable can also support conversations between transaction programs located at the same LU (option set 204).