The system ensures that the data set labels on prior volumes do not have the last-volume indicator on. The volume with the last-volume bit on is not necessarily the last volume that contains space for the data set or is indicated in the catalog. A later volume might also have the last volume bit on.
When you later extend the data set with DISP=MOD or OPEN with EXTEND or OUTINX, OPEN must determine the volume containing the last user data.
With a system-managed data set, OPEN tests each volume from the first to the last until it finds the last-used volume.
With a non-system-managed data set, the system follows a different procedure. First OPEN tests the data set label on the last volume identified in the JFCB or JFCB extension. If the last-volume indicator is on, OPEN assumes it to be the last-used volume. If the indicator is not on, OPEN searches the volumes from the first to the second to last. It stops when it finds a data set label with the last-volume indicator on. This algorithm is for compatibility with older MVS levels that supported mountable DASD volumes. If this algorithm is unacceptable, you can either delete the data set from all volumes or delete the data set from the volume that has the last-volume indicator on.