The application program uses the OPEN macroinstruction, which points to an ACB, to formally present itself to VTAM®. After this macroinstruction completes successfully, the application program can request VTAM services.

More than one ACB can be opened by an application program. This means that an application program that performs more than one function (for example, communicating with logical units and acting as a program operator application program) can be defined so that it is viewed by VTAM as being more than one application program. However, most VTAM users will find it satisfactory to open only one ACB for each application program.