FrameMaker + SGML ("F+S") is a full featured SGML editor. It allows you to import SGML DTDs and text and to export the same. XML imports and exports also work (but with some restrictions). F+S is quite easy to learn at the user level, but very difficult if you want to define your own SGML application. So if you are alone with your F+S Box, plan a week to get really going unless you just want use Frame's version of Docbook. If you simply want to edit with Docbook or lternatively use something your local SGML guru set up for you, you are in business after an hour or two and this document will help you just with that. For people who'd like to import XML DTDs into F+S, see my other quick guides.
F+S has its own element description language with which "SGML engineers" define EDDs (Element Definition Documents). EDDs define the structure of a document, most of its layout and more advanced things (like cross-reference elements, elements that act as index markers, etc.). When you author a document, you simply start with a blank template and then import definitions from an EDD files if the template does not have them already. The EDD also contains a reference to an SGML application, i.e. a short entry in some file somewhere on your system hat defines things like the character sets you use, (read/write rules form SGML/XML to F+S and the other way round), SGML configuration (important for XML fans) and such.
In other words, you can not just "use" FrameMaker+SGML besides DocBook: You either need some technical support in your site or you have to know how to register SGML applications for EDD files you can find elsewhere. Along with FrameMaker you will have an old version of the large DocBook schema and some lighter ones used in some demos. Both are difficult to find on all architectures.