My first thought was “which of these game effects or modifiers stack with each other?” And then I thought about a stack as a term for a group of counters in a war game. Which started me thinking how my gaming context, born of my own experience, is subjective – I doubt any of the library games group would have thought that way given this prompt. A couple of times in my recent D&D campaign, I put the players in quite “wargamey” situations and they resolved them quite differently than I might have. A reminder that there is no “one way” to play and run RPGs, despite what old grognards like me might think.