IMAGE mode was used to create an image of the GHRS aperture being used. Obviously this would be a very small region of the sky, but it would be taken in the ultraviolet, and it would show how the light of the object was distributed in the aperture, which was sometimes helpful for after-the-fact analysis of what was observed. A similar mode was invoked if a MAP were requested. GHRS acquisitions are explained in "Understanding GHRS Acquisitions" on page 35-19.
ACCUM mode was by far the most commonly used mode of the GHRS as it allowed the use of all the automatic procedures for optimizing the data acquisition (FP-SPLIT, substepping, and comb addition-see "Detectors" on page 34-7). The calibrated data (.c1h and .c0h files) for a typical ACCUM mode observation will have multiple groups. Each of these groups is an independent (sub)integration and they must be added together (realigning the data in wavelength space before adding) to produce the total calibrated spectrum for your observation. See "Assessing GHRS Science Data" on page 35-29 for examples of ACCUM data and a description of how to reduce and analyze such data.
WSCAN was a variant of ACCUM in which one could specify a range of wavelengths to be observed and the software would determine how to space grating movements to accomplish that. OSCAN was another variant of ACCUM that applied only to the echelle gratings (scanning through orders); it was used almost exclusively for calibrations.
34.3.3 RAPID Mode
For observations of targets whose spectra varied on shorter timescales, there was rapid (or direct) readout mode. In RAPID mode, individual integrations could be obtained as often as once every 50 milliseconds. However, RAPID mode did not allow use of the FP-SPLIT or substepping producedures and observations of the detector background were not interspersed with the on-target observations. In RAPID mode data, each subintegration was stored sequentially in separate groups in both the raw and the calibrated data.