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