User sessions are called LU-LU sessions in both subarea and APPN networks. An LU-LU session between two logical units enables one user to communicate with another. An example of an LU-LU session is an application, such as CICS®, communicating with a workstation. LU-LU sessions are used only for user data, not for network control data.

A VTAM® node can both initiate sessions and respond to session initiation requests. An LU-LU session is established when a BIND is sent from the primary LU to the secondary LU. Dependent LUs act as secondary LUs only and have an LU-LU session limit of one. Independent LUs can have parallel sessions between the same pair of LUs and can have multiple sessions between one LU and several other LUs. Independent LUs exchange only UNBIND and RSP(UNBIND) for session termination. Independent LUs can be either the PLU or the SLU for any given session. An UNBIND can be sent by either LU.

In an APPN network, the owning CP for an LU uses CP-CP sessions to obtain location and routing information needed to establish an LU-LU session. The owning CPs of the session partners do not have to be adjacent to each other and do not require having a direct CP-CP session with each other if they are not adjacent.

Sessions are established using a specific mode. The mode identifies session parameters and the APPN Class of Service (COS) name to use for the session. In subarea networks, the Class of Service indicates the virtual route and transmission priority to be used for this session. In APPN networks, the APPN Class of Service tables specify weights to be applied to the various routes in the APPN network based on the characteristics of the links and nodes. These weights are calculated, taking into account the various characteristics such as acceptable level of security, cost per byte, cost per connect-time, propagation delay, and effective capacity. For more information about Class of Service in APPN, see APPN class of service.

VTAM allows dependent LUs, such as workstations and printers supporting LU types 0, 1, 2, and 3 in one VTAM domain to access VTAM application programs in a different domain by using APPN protocols between domains. All dependent LU types are supported through APPN networks with one exception: Sessions with binary synchronous 3270 devices are not supported over APPN links.