This is from the tabletop game Warhammer 40k rulebook:

The turn Marines arrive they may not assault.

I always assumed it means they are not allowed to assault. But the meaning "they may choose not to assault" (as opposed to "they must always assault") would be perfectly legitimate too, as far as game rules go.

So how do you tell whether it limits your choices (not allowed to) or extends them (allowed not to)? How it should be worded if the meaning was "allowed not to"?