Certain abbreviations are used pervasively throughout the GHC source code. A few, like "Tc", are easy to figure out. Others, like "Occ", are more challenging (in case you were wondering, none of GHC's code is  Occult, except perhaps in the Latin sense)

* "Occ" means "Occurrence"

* "Rn" means "Renamer"

* "Rdr" means ??? (help, somebody fill this in, I still have no idea what it stands for)

* "Tc" means "TypeCheck?{ing,er}"

* "Lcl" means "Local"

* "Gbl" means "Global"