All reason codes are documented in the mfpubcat.cpy copybook, in a section labeled reason codes. For example:
...
*=================================================================
* reason codes
*=================================================================
78 78-REC-NOT-FOUND VALUE 001.
78 78-INVALID-DYN-PDS VALUE 002.
78 78-INVALID-CATALOGED-FLAG VALUE 003.
...
Match the returned reason code value to that of a 78-level field in the copybook. The field name provides the reason for the code.