User generated content I assume. Anyway disabling UGC substitutions will mean no user created liveries or formations will appear on AI trains on routes you play.
User-Generated Content, ie mods. I assume it refers to liveries and formations from Creator's Club/Livery Editor/Formation Editor.
those 2 are for the Creators Club (liveries / scenario creations / formations) Creators Club allow mods is for mod creators (pc only) mods can now be placed into Documents\my games\TrainSimWorld5\Saved\UserContent (can be different if using something like One Drive)