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A VTAM® application program can contain two types of instructions: communication instructions and processing instructions. The application program always contains communication instructions. These instructions send and receive SNA messages, called request/response units (RUs), and control other aspects of communication between the application program and other elements in the network. The application program can also contain processing instructions. These optional instructions manipulate data before it is sent or after it is received.

If a VTAM application program is small and contains processing instructions, those processing instructions can be blended with communication instructions. More commonly, however, the processing instructions are written separately, with an interface defined between one or more processing parts of the program and the communication part of the program. This separation of function enables each part to be created separately and means that changes or additions to one part do not affect other parts.

VTAM application programs can share the resources of the SNA network; that is, multiple application programs can use the same communication controllers, cluster controllers, and communication lines to reach LUs. For example, in Figure 1, VTAM application programs A and B use the same communication controller (at 5) and the same SDLC links to reach LUs (at 6). The application programs, however, are not aware that they are sharing these resources because VTAM, the network control program (NCP), and other programming elements in the network handle communication in such a way that the programs do not know they are sharing the resources.