Pointing origins (POs) are calibrated positions within the focal plane of the instrument. Each instrument has at least one, and often several, POs. One might be the center of the science detector, another might be the center of the slit, and a third might be the center of the guider.
The pointing origins are defined in millimeters in the rotator frame, with (0,0) being the rotation axis. Instruments without a rotator use some arbitrary definition of (0,0).
The angle of the coordinate system is usually defined by having "up" on the guider camera be the elevation axis when the rotator is set to physical position angle 0. With the zero-point and the rotation of the system defined, all other relationships between pointing origins are known.
KSD 40 is a reference which describes in detail the definition of POs and the relationships between various coordinate systems. It also contains instrument-specific details about POs.