The Domain attribute also controls how Frames align with each other. If the Domain value in a Frame is set, then only Frames with the same Domain value can be aligned with it.
Some Frames are given standard Domain values when they are created (e.g. GRID, FRACTION, PIXEL, AXIS, SKY, SPECTRUM, CURPIC, NDC, BASEPIC, GRAPHICS). Frames created by the user (for instance, using WCSADD) can have any Domain value, but the standard Domain names should be avoided unless the standard meanings are appropriate for the Frame being created.
KAPPA --- Kernel Application Package