DCB pointer

z/VSE®
This field is a fullword and always contains zeros. It is included to maintain compatibility with the Exit Parameter List in the z/VM® and z/OS® environments.
z/VM and z/OS
The DCB pointer is a fullword that contains the address of the Data Control Block.
The assembler sets this address which points to the applicable DCB for the exit being called as follows:
Exit DCB
SOURCE SYSIN
LIBRARY SYSLIB
LISTING SYSPRINT
PUNCH SYSPUNCH
OBJECT SYSLIN
ADATA SYSADATA
TERM SYSTERM

When an exit is invoked with an OPEN request, the data set referred to by the DCB is not open, and the contents of the DCB might not be complete.

When an exit is invoked with a PROCESS request, the exit might use the DCB to obtain additional information about the data set or member being used. For example, on z/OS, the exit can obtain user information from a PDS or PDSE directory by using the BLDL system macro.