Epson 386SX - Manual (Page 106)

The extension is optional and can be up to three characters
long. You can use the extension to further identify a file or to
describe what type of file it is, such as a text file or program file.
When you use an extension, separate it from the filename with
a period, like this:
DATA.TXT
Do not use uppercase and lowercase letters to distinguish
between files. MS-DOS does not recognize the difference and
displays all filenames in uppercase.
Some application programs automatically add extensions to the
files you create. These programs use the extension to determine
whether a data file is compatible. Avoid using the same
extensions that your application programs use.
MS-DOS reserves certain filenames for its own use. The
reserved filenames are:
AUX
CLOCK$
COM1
COM2
COM3
COM4 LPT3
CON LST
LPT1 NUL
LPT2 PRN
MS-DOS also reserves certain extensions for program files. The
reserved extensions are .COM, .EXE, and .BAT, and files with
these extensions are also sometimes called executable files. Do
not use these reserved filenames and extensions for your data
files.
The extension .BAT denotes a type of executable file called a
batch file. You can use batch files to automate sequences of
MS-DOS commands. Even if you are not a programmer, you
may want to create some batch files to save time. See “Creating
an AUTOEXEC.BAT File” in this chapter for a description of a
particularly useful kind of batch file, an autoexecute batch file.
4-10 Using MS-DOS with Your Equity 386SX