You can include additional PL/I files at specified points in a compilation unit by using %INCLUDE statements. For the %INCLUDE statement syntax, see PL/I Language Reference.
If you specify the file to be included using a string, the compiler searches for the file exactly as named in that string. If you specify an include file using one of the more traditional PL/I methods, however, by either using a ddname and member name or just a member name, the compiler appends a file extension to the member name.
You can specify which file extensions are appended to the member name by using the INCLUDE compiler option. For example, if you specify the INCLUDE option as INCLUDE(EXT(CPY)), when the compiler sees either of the following statements, it tries to include the file member.cpy.
The compiler searches for this file in the following order:
If you specify more than one extension in the INCLUDE compiler option, the compiler searches all the directories above using the first extension; then does another pass through all the same directories using the second extension, and so on.