OpenType Feature File Specification | Adobe Developer Connection
http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html
An OpenType feature file is a text file that contains the typographic layout feature specifications for an OpenType font in an easy-to-read format. It may also contain override values for certain fields in the font tables. It is read in during the creation or editing of an OpenType font. This document specifies the feature file grammar.