SGM

The sgm file extension is associated with Visual Boy Advance, one of several emulators to emulate the Nintendo Gameboy Advance handheld console.

The sgm file contains saved state of game (save).

EAD DTD is designed to function as both an SGML and XML DTD.

It conforms to all SGML/XML (ISO 8879) specifications. It has been thoroughly tested using a wide variety of existing SGML/XML software. To be used as an XML DTD, "switches" have been included in the DTD for turning off features used only in SGML applications, and turning on features used in XML applications. Instructions for using these "switches" are contained in the DTD itself.

Please keep in mind, downloading the EAD DTD and related files is not all you will need to do to begin using it. Most SGML and XML authoring software requires more than simply loading a new DTD to make them work since there are often proprietary features and add-ons to contend with. You will most likely need to consult manuals or user guides on your SGML/XML software after getting a copy of the EAD DTD and related files themselves.

The SGM file extension is also used for files written in the Standard Generalized Markup Language -  IETF document.

The Standard Generalized Markup Language (ISO 8879:1986 SGML) is an ISO Standard metalanguage in which one can define markup languages for documents. SGML is a descendant of IBM's Generalized Markup Language (GML), developed in the 1960s by Charles Goldfarb, Edward Mosher and Raymond Lorie (whose surname initials were used by Goldfarb to make up the term GML).

SGML provides an abstract syntax that can be implemented in many different concrete syntaxes. For instance, although it is the norm to use angle brackets as tag delimiters in an SGML document (per the reference concrete syntax defined in the standard), it is possible to use other characters instead, provided that a suitable concrete syntax is defined in the document's SGML declaration. For example, an SGML interpreter could be programmed to parse GML markup.

MIME: text/sgml, text/x-sgml, application/sgml, application/x-sgml

The sgm file extension is also associated with the XMetaL, a specialized XML editor for structured authoring from JustSystems available for Microsoft Windows.

Most likely it is the programs default file format.