SpellNoDec casts a spell using all legal means (casting time, interruptable), but without requiring memorization. The RES bit, as with all other RES actions, means that it takes a SPL file as an argument rather than an IDS entry.
I saw this coded in a .BAF file and tried to look it up in TeamBG's IESDP's Actions list for BG2 but they don't give an explanation as to what this does.