If the ClipOp attribute of a Plot is zero (the default), graphical output is visible only if it satisfies the clipping limits on all the axes of the clipping Frame (a boolean AND). Otherwise, if ClipOp is non-zero, output is visible if it satisfies the clipping limits on one or more axes (a boolean OR).
An important use of this attribute is to allow areas of a Plot to be left clear (e.g. as a background for some text). To achieve this, the lower and upper clipping bounds supplied to AST_CLIP should be reversed, and the ClipOp attribute of the Plot should be set to a non-zero value.
AST A Library for Handling World Coordinate Systems in Astronomy