DESI data are grouped broadly by category, using environment variables to define the base directory under which files of that category are kept. These variables have a standard location relative to DESI_ROOT, but code uses these variables so that one can swap out different input/output locations for testing.
$DESI_ROOT : represents the top level of the DESI directory tree. <DESI_ROOT/index> $DESI_SPECTRO_DATA : raw data <DESI_SPECTRO_DATA/index> $DESI_SPECTRO_REDUX : processed spectro data <DESI_SPECTRO_REDUX/index> $DESI_SPECTRO_SIM : simulated spectro data <DESI_SPECTRO_SIM/index> $DESI_SPECTRO_CALIB : spectro calibration data <DESI_SPECTRO_CALIB/index> $DESI_SURVEYOPS : data files used for day-to-day survey operations <DESI_SURVEYOPS/index> $DESI_TARGET : target selection and fiber assignment (see also Myers et al. 2023) <DESI_TARGET/index> $DESIMODEL : data used for simulating DESI <DESIMODEL/index> $DESISURVEY_OUTPUT : outputs from desisurvey and surveysim <DESISURVEY_OUTPUT/index> $PROTODESI : data and logs from the ProtoDESI campaign <PROTODESI/index>