Application programs must use an access method control block (ACB) to identify themselves to VTAM®. The ACB points to a location in the program that contains the name of the application program as specified in an APPL definition statement during VTAM definition. It can also point to an EXLST control block containing the addresses of exit routines that are to be associated with the application program.

The application program name pointed to by the ACB corresponds to the LU name that another VTAM application program would specify if requesting a conversation with one of the application program's transaction programs.

The application program must create the ACB and initialize the fields within it. VTAM provides a macroinstruction, ACB, that allows application programs to build ACBs during program assembly.