The OPEN macroinstruction associates an application program with VTAM® so the application program can use VTAM facilities. The OPEN macroinstruction specifies an ACB; the ACB, in turn, 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. The ACB can also point to an EXLST control block containing the names of exit routines that are to be associated with the application program. An EXLST can also be pointed to when a session is established with an LU. See Establishing sessions with LUs in this chapter. When the open process has completed, exit routines specified in the EXLST are eligible for scheduling by VTAM.

Because a program can open multiple ACBs, a program that performs related functions (for example, communicating with LUs or acting as a program operator application program) can be defined in such a way that VTAM views the program as being more than one application program. However, most VTAM users might find it satisfactory to open only one ACB for each program.