Every FrameSet has a "base" Frame and a "current" Frame (which are allowed to be the same). Any of the Frames may be nominated to hold these positions, and the choice is determined by the values of the FrameSet's Base and Current attributes, which hold the indices of the relevant Frames. By default, the first Frame added to a FrameSet is its base Frame, and the last one added is its current Frame.
The base Frame describes the "native" coordinate system of whatever the FrameSet is used to calibrate (e.g. the pixel coordinates of an image) and the current Frame describes the "apparent" coordinate system in which it should be viewed (e.g. displayed, etc.). Any further Frames represent a library of alternative coordinate systems, which may be selected by making them current.
When a FrameSet is used in a context that requires a Frame, (e.g. obtaining its Title value, or number of axes), the current Frame is used. A FrameSet may therefore be used in place of its current Frame in most situations.
When a FrameSet is used in a context that requires a Mapping, the Mapping used is the one between its base Frame and its current Frame. Thus, a FrameSet may be used to convert "native" coordinates into "apparent" ones, and vice versa. Like any Mapping, a FrameSet may also be inverted (see AST_INVERT), which has the effect of interchanging its base and current Frames and hence of reversing the Mapping between them.
Regions may be added into a FrameSet (since a Region is a type of Frame), either explicitly or as components within CmpFrames. In this case the Mapping between a pair of Frames within a FrameSet will include the effects of the clipping produced by any Regions included in the path between the Frames.
AST A Library for Handling World Coordinate Systems in Astronomy