Hi,
Think of facets as a little piece of metadata you can attach to a single value. So, if your triple is part of a list type, each value in the list can have each facet once (by name). Read more about facets on lists here.
Facets cannot be indexed, they will always be a value lookup to access/filter by.
Facet types are guessed from the RDF input based on their format(eg: false is a bool type, "mything" is a string, etc. code that does this is here). However in using the gRPC api they can be set explicitly (eg: dgo)
Make sure you read up on facets in general here. Strongly consider the warning:
Though you may find yourself leaning towards facets many times, they should not be misused…