We have a few bugs open that call for functionality to select entities by a given selector string (a sitelink or similar things) in wiki pages (by using Lua or even a parser function).
In order to provide that, we need a way to track such usages and purge in case something relevant changes. The current usage tracking can't handle this, because it can only track usages of entities by entity id. Unless we consider these cases negligible, we will need new usage tracking functionality for selectors that don't match an entity, thus the selector itself needs to be tracked, so that we can purge once the selector matches an entity.
Also this will be relevant should we not restrain the suggested lookups to only ever return one item per selector.
Usage tracking data for that would probably look like: (page id, selector type, selector value)