In a rather similar way, spatial position is given as x,y offsets relative to a reference frame defined by AXIS_RA, AXIS_DEC and POSITION_ANGLE. These might define the frame of the instrument during the observation, but in general (e.g. for a scanning instrument) they just specify some convenient reference frame.
Note that FIELD_RA and _DEC do NOT define the reference frame for the position information; they just contain the coordinates of the field centre.