The type of calibrations used depend, obviously, on the instrument and the data reduction recipes used. Typically there are three kinds of calibration frames:
Library frames provided by the observatory (bad pixel masks, rotation transformations, etc). These are maintained by the instrument scientist as appropriate. They are located in ORAC_DATA_CAL.
Nightly frames that are generated during observing. The may be taken in specific calibration observations, e.g. by taking a ``dark'' (at UKIRT) or ``skydip'' (at JCMT) frames. They might also be generated from actual observations of targets (such as ``sky flats'') or calibration values (such as ``flux conversion factors'') calculated as part of a recipe. These are located in ORAC_DATA_OUT.
``Rule'' files that contain the rules for what constitutes an appropriate calibration frame. These are located in ORAC_DATA_CAL.
ORAC-DR treats the first two kinds rather differently.
Library frames reside ORAC_DATA_CAL and their selection is hardwired either in the instrument class or in a DR primitive. The users are unlikely to be concerned with them unless they want to override them with their own.
Nightly frames are handled in a more complicated way. A DR recipe that generates a calibration frame is responsible for filing it with the pipeline. The pipeline will hand it back to recipes that require calibration recipes according to a set of rules that are defined on a per-instrument basis by the ORAC-DR infrastructure as well as a per-frame basis by the calibration rules files.
ORAC-DR: Overview and General Introduction