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

ON-units for qualified and unqualified conditions

There can only be one established ON-unit for an unqualified condition at any given point in a program, but there can be more than one established ON-unit for qualified conditions. For example, in handling the ENDFILE condition as qualified for different files, you can have an ON-unit established to uniquely handle the occurrence of ENDFILE for any one of the files.

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