These are facets that AS-Tools knows about and may wish to have customized responses for. For example, we may wish to customize the list of jars on a classpath more deeply for facets we know about, and may not choose to customize for facets we know nothing about. So in this case, if a given facet is in our KNOWN_FACETS list, we may choose to customize it, while we don't even bother for a facet like wonka.max.