mtl is the root directory for the Merged Target List ledgers. The MTL ledgers record the state of each DESI target as redshift information is acquired. There is a set of ledgers for each of phase 2 of Survey Validation (sv2) the One-Percent Survey (sv3) and the Main Survey (main).
The mtl-done-tiles.ecsv and scnd-mtl-done-tiles.ecsv files in the mtl directory track which tiles have had target states updated over the course of all DESI survey phases (i.e. all of sv2, sv3 and main).
Files and subdirectories:
mtl-done-tiles.ecsv : Record of updated tiles for primary targets <mtl-done-tiles> scnd-mtl-done-tiles.ecsv : Record of updated tiles for secondary targets <scnd-mtl-done-tiles> main/index sv2/index sv3/index