A PDSE can have up to 123 extents. Because a PDSE can have more secondary extents, you can get the same total space allocation with a smaller secondary allocation. A PDS requires a secondary extent about eight times larger than a PDSE to have the same maximum allocation. Conversely, for a given secondary extent value, PDSEs can grow about eight times larger before needing to be condensed. Defragmenting is the process by which multiple small extents are consolidated into fewer large extents. This operation can be performed directly from the interactive storage management facility (ISMF) data set list.
Although the use of smaller extents can be more efficient from a space management standpoint, to achieve the best performance you should avoid fragmenting your data sets whenever possible.