PE = Polyethylene
PET = Polyethyleneterephtalate (=Polyester)
RC = Resin Coated (here in the meaning of laminated with plastic foil; to my understanding only PE foil has been used for this.)
Thus you have prints consisting of
-) a plain paper base, with some baryta coating, (called `fibreŽ),
-) a base from paper laminated on both sides with PE-foil (called `RC-paperŽ)
-) and a base from a plain PET sheet.