This is a 2D coordinate system conceptually inscribed on the instrument module in the focal plane. Each focal-station has such a coordinate system, although it is currently explicitly used only at those that have image rotators. As can be seen from See Telescope coordinate systems looking towards sky (excludes Nasmyth foci) , the pointing origin coordinate system is right-handed when looking towards the sky.
For focal-stations with rotators, the origin of the coordinate system is defined as the point at which the rotator's rotation axis intersects the focal plane, and the Y axis is parallel to the TV Y axis. The rotator zero point is chosen so that at zero rotator angle (and, for Nasmyth foci, elevation angle) the pointing origin Y axis coincides with the telescope elevation axis.
For focal-stations without rotators, pointing origin offsets are indistinguishable from collimation corrections and the choice of coordinate system origin is arbitrary. It will typically be the center of the guider detector.
(X,Y) positions are measured in mm (to underline the fact that a pointing origin is a position in the focal plane). Pointing origins are identified by name and the REF pointing origin is always the center of the guider (for LRIS, which has two guiders, it is the center of the slit-viewing guider). The notation (XIM,YIM) refers to a point in the pointing origin coordinate system ("IM" for "IMage plane").
The term "pointing origin" is widely used but is nevertheless the source of much confusion: the name does not imply what the coordinate system is used for. Accordingly, while the name is being retained, user software will increasingly use the synonymous and more descriptive term "detector location".