Sometimes it would be great to have some additional data about the overall results returned by an api resultset query. For example, if an API query returns a list of things that include a library they're attached to, it might be nice to be able to get a list of all libraries that appear in the resultset. (not just the current page). This way, we could display a select filter of just the libraries associated with the current api request to allow further filtering.. i.e. facetting. We floated the idea of x-koha-facets for requesting this, but we haven't yet come up with a good way to get this data or return it.
The `status+strings` embed option in ILLBackends as actually really a facet.. we should move it to a new syntax once we've agreed upon it asap.