Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide

(NO)DESCRIPTOR

The DEFAULT(DESCRIPTOR) option indicates that, by default, a descriptor is passed for any string, area, or aggregate parameter. However, the descriptor is used only if the parameter has nonconstant extents or if the parameter is an array with the NONCONNECTED attribute.

In this case, the instructions and space required to pass the descriptor provide no benefit and incur substantial cost (the size of a structure descriptor is often greater than size of the structure itself). Consequently, by specifying DEFAULT(NODESCRIPTOR) and using OPTIONS(DESCRIPTOR) only as needed on PROCEDURE statements and ENTRY declarations, your code runs more optimally.

Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide