SURVEY
The SURVEY is the phase of the overall DESI project the spectra were
observed in. For example, SV1, SV2, SV3, main, following
the naming convention used in this set of files.
The descriptions here and below reflect the main survey. The intermediate files
for SV1, SV2, SV3, special are simlar to main, but the directory
structures were somewhat different and reflect an evolving situation.