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

KEYLENGTH

The KEYLENGTH option specifies the length, n, of the recorded key for a KEYED file. You can specify KEYLENGTH only for INDEXED files (see ORGANIZATION later in this section).

Read syntax diagramSkip visual syntax diagram>>-KEYLENGTH--(n)----------------------------------------------><
 

If you include the KEYLENGTH option in a file declaration, and the associated data set already exists, the value is used for checking purposes. If the key length you specify in the option conflicts with the value defined for the data set, the UNDEFINEDFILE condition is raised.

ISAM and BTRIEVE
Keys are kept in the index pages of an ISAM or BTRIEVE file. The length of the key needs to be defined to PL/I when the file is created.
Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide