If a location says that cards cannot be "added", it's going to prevent anything from ending up there - you can't move [card name="Nightcrawler"] there, [card name="Squirrel Girl"] won't put any squirrels there, and many more. However, if a location says card cannot be "played" here, you simply can't play cards from your hand there, you'll still be able to move your [card name="Nightcrawler"] into it, or send a tiger over there with [card name="White Tiger"].