The SCREEN subroutine determines whether the RLSE or SQTY subroutines should be run. DASD sequential and partitioned data sets being processed by BSAM or QSAM and opened for OUTPUT or OUTIN are selected. The VTOC and data sets starting with SYS1. are excluded. You can write IFG0EX0B to make further selection tests or add other support, for example:

• You can handle secondary allocation differently for an sequential extended-format data set. They can have up to 123 extents per volume, whereas physical sequential data sets can have up to 16 user data extents.
• Default the number of buffers larger if the data set is an sequential extended-format data set.
• For a compressed data set, a spooled data set, dummy data set, or z/OS UNIX file it is not useful to override the QSAM BUFNO default of 1. The extra buffers are not used productively. If you know the application program has no dependence on the number of buffers, your exit can decrease a user-provided BUFNO.