DDNAME Passed to Checkpoint

If your program provides a DDNAME instead of a DCB, the checkpoint routine opens the checkpoint data set the first time it receives that DDNAME.

Sequential data sets
For a sequential checkpoint data set, the data set's disposition controls where the first checkpoint is written. If the data set's disposition is NEW or OLD, the first checkpoint entry is written at the beginning of the data set, even if a previous checkpoint call has written an entry. If the data set's disposition is MOD, the first checkpoint entry is written after the last entry existing in the data set. DISP=OLD, NEW, or MOD has no effect on where checkpoint entries other than the first are written. Each following entry is written after the previous entry.
PDSs
PDSs are treated in the same manner as nonopened DCBs passed to checkpoint.

Note: If your program uses more than one DDNAME on executions of the CHKPT macro, the data set for the previous DDNAME is closed and the data set for the new DDNAME is opened.