A Frame is used to represent a coordinate system. It does this in rather the same way that a frame around a graph describes the coordinate space in which data are plotted. Consequently, a Frame has a Title (string) attribute, which describes the coordinate space, and contains axes which in turn hold information such as Label and Units strings which are used for labelling (e.g.) graphical output. In general, however, the number of axes is not restricted to two.
Functions are available for converting Frame coordinate values into a form suitable for display, and also for calculating distances and offsets between positions within the Frame.
Frames may also contain knowledge of how to transform to and from related coordinate systems.
AST A Library for Handling World Coordinate Systems in Astronomy