A-Shell/FAX is an application programming interface (API) which enables applications running under A-Shell/Windows to send faxes via the Windows Fax Service. While faxing has always been possible under A-Shell/Windows by the simple fact that most fax packages emulate a standard Windows printer, A-Shell/FAX allows the application to pass all of the necessary addressing information (fax number, to/from names, cover page info, etc.) directly to the Fax Service, eliminating the need for the user to respond to the dialog boxes which would otherwise pop up to request this information.

Note that A-Shell/FAX requires Windows XP or later, although it can be made available to A-Shell/UNIX via ATE and the GDIPRT printing subroutine.

The mechanism for passing this information from the application to the Fax Service consists of a set of commands or directives that can be embedded at the start of the print file. The concept is identical to that used for embedding GDI directives for font selection, line drawing, image printing, etc. The actual set of directives overlaps that of the AlphaFAX package available under AMOS. That is to say, most (but not all) AlphaFAX commands are supported by A-Shell/FAX, yet A-Shell/FAX supports a number of additional commands designed to take advantage of features of the Windows Fax Service that do not exist in AlphaFAX.