Aperture is the generic term used for any targetable area in a Science Instrument (SI), a small slit, an occulting finger, different points on a CCD, a virtual aperture that encompasses a number of chips (e.g. WFALL), anything that can be specified in the APERTURE field of the proposal file, and a few more that can't. The scales, orientations, locations in the HST FOV, and a number of other properties of these targetable areas are calibrated and updated periodically. Though these positional calibrations largely fall under the calibration plans of the Science Instrument Groups, some special calibrations are made by the Observatory Support Group, which also maintains the aperture data and relations. This information, in the form of the Science Instrument Aperture File (SIAF), is installed into the HST Project Data Base whenever updates are calculated and needed.
For more specific information on HST Apertures, please see the HST Apertures Documents page.