ISL could stand for "Island Simulation," and LS might be "Life Simulator" or "Lost Survivors," but that's speculative. "Full File" might refer to a complete scenario or campaign, number 29 in a set. "Set Txt" suggests it's a text file set, which could be mission descriptions, scenarios, or story elements.
, these files may be used to handle custom text for game sections. : This specifically designates LEGO Island 2: The Brickster's Revenge ISL LS Island 2 Full File 29 Set Txt
What are you trying to load these files into? ISL could stand for "Island Simulation," and LS