File names have three parts, the name, extension, and directory name.
Extensions are 0 to 3 letters or digits.
The file extension is often used to identify the different kinds of files.
Some common extensions and their meanings are:
REL	relocatable binary file
DMP	core image file
F4	Fortran Source
SAI	Sail source
FAI	FAIL source
LST	listing output file
DOC	some documentation file
MSG	MAIL message file
PUB	PUB source file
XGP	file prepared for Xerox Graphics Printer
FNT	XGP font file
TMP	temporary file for some program
RPG	information saved from RPG commands (COMPILE, etc.)
TXT	ASCII text file other than above

On the program documentation areas [S,DOC] and [UP,DOC], the file
extension is usually used to indicate the programmer name of the
author of the file.