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

(RE)ORDER

The DEFAULT(ORDER) option indicates that the ORDER option is applied to every block, meaning that variables in that block referenced in ON-units (or blocks dynamically descendant from ON-units) have their latest values. This effectively prohibits almost all optimizations on such variables. Consequently, if your program logic allows, use DEFAULT(REORDER) to generate superior code.

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