Well tpoc says it all I am trying to find a way to make a magic effect disabled in interiors but working in the exteriors of towns (which are treated by the game as "interior"): How do I need to modify the "if interior" code?

You could make a form-list of all the interiors (that are exteriors) you want to capture and check (in script) whether the player is currently in one of the cells in your (form)List?

there's a condition funciton for this. goes like "isininterior" or something