ch = Prefix for Character .pac files. XXX = placeholder for the three digit Wrestler ID (also sometimes referred to as PACH ID, SlotID or the likes) the .pac file represents. Y = placeholder for the one digit Attire Number the .pac file represents. Z = placeholder for the one digit Attire Type the .pac file represents.
The above are mandatory for the naming scheme of chXXXYZ.pac files, although you may also include a suffix that doesn't contain any digits. Click the links to learn more about valid values, etc..
The placeholders have to be replaced with the appropriate digits, of course.
Wrestler ID - Explanation of the Wrestler ID
Attire Number - Explanation of the Attire Number
Attire Type - Explanation of Attire Type