A facet is a group of headings which all define a certain method of classification. That is, a facet is a way in which a resource can be classified; for example, classified by colour, classified by geography, classified by subject, etc.The facet called 'Organisation' is definitely a group of headings, and they seem to classify a resource in the same way, i.e. it is a set of organisations, not other things. The trouble though with facets is that facets are close to faceted classification, which is a different kind of species, not necessarily related to facets at all. Faceted classification implies facets, but facets do not imply the usage of a faceted classification scheme for classification.
Faceted Classification: In a strict faceted classification model, a resource is classified under one heading from each facet that applies to it. A resource does not have to be classified at all in a given facet, if that facet's method of classification doesn't apply to the resource.So what we can determine is that fac.etio.us uses facets, but has no faceted classification as a scheme for classification. If that strict model were applied it may not be possible to classify an object with more than one heading from the same facet. If we would consider facets in fac.etio.us being parts of a faceted classification, then a facet would be:
A set of headings in which the assignment of one heading to a resource limits the assignment to that resource of other headings in the set.This is not the case in fac.etio.us.