We call them cache keys. We use a hash of entirely arbitrary data you want a cache item to invalidate on when that data changes. For example, a cached snippet

We call them cache keys. We use a hash of entirely arbitrary data you want a cache item to invalidate on when that data changes. For example, a cached snippet of HTML for a related products section should invalidate if the product IDs change, so you hash the Item ID with the product IDs and bam, you got a cache key that will let you know when you need to reprocess that snippet. Put that bad boy as a child of another snippet and you can cache the whole lot. It's cache keys all the way down! Caveat: not worth the engineering effort for simple systems, just use an FPC and call it a day.