In supporting LU 6.2, VTAM® uses the concept of finite state machines to describe the condition of one side of a conversation at any given time. The LU 6.2 architecture defines what transaction programs can do in any given state. Consequently, application programs are restricted to using a subset of the APPCCMD macroinstructions at any given time. For example, an application program that has been asked to respond to a confirmation request must use one of the APPCCMDs appropriate to a confirmation response. VTAM rejects any other macroinstructions. For more information on conversation states, see Maintaining conversation states.