Currently facets only apply to hosts.
However, there is a lot in common between hosts and host groups - in attributes, in forms, etc.
The differences between hosts and host groups are also common to all facets - such as having hosts inherit attributes from host groups, attributes are only required on hosts and not on host groups etc.
I would like to be able to write a facet once and have it added to both host and host groups, without having to repeat my code and with the common changes depending on the object being handled automatically.