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

Using keys with regional data sets

Source keys are used to access REGIONAL(1) data sets. A source key is the character value of the expression that appears in the KEY or KEYFROM option of a data transmission statement to identify the record to which the statement refers. When you access a record in a regional data set, the source key is the region number.

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