The FSIO API is an abstraction layer for dealing with an underlying filesystem. Initially intended as an abstraction behind which implementations for Unix and Windows filesystems could be transparently used, this layer also allows a great deal of control of what the underlying filesystem is, what it looks like to the user and how it operates.
This layer was structured so that third-party modules could easily declare, or register, a set of custom filesystem functions and attach those functions at particular points in the filesystem. To do this, there are functions available for modules' use and a structure representing a filesystem handler or object.
The FSIO module developer should read the description of the filesystem object first, then the functions for working with these objects. There is also a development document that contains some suggestions and notes to keep in mind when devising one's own FSIO modules.