Concepts and collections

Hello, I have a query about what is a concept and what is a collection and
if an item can be both?

We've published a "Circumstance List" as a skos:ConceptScheme with a URI of
http://id.esd.org.uk/personCircumstanceList/1.00 . That redirects to a web
page which illustrates its hierarchical structure.

Each top level heading (currently defined as a skos:Concept) defines a
demographic grouping like, "Age", "Addict/abuser", "Ethnic origin".  Second
level headings are individual specific circumstances like specific ages,
ethnic groups or types of Addict (smoker, drinker, ...).

I _think_ that "Age" and "Ethnic origin" are not actually circumstances
themselves and so might rightly each be just a skos:Collection.  It would
present a problem maintaining the hierarchy if some terms top level headings
were not skos:Concepts.

I think "Addict/Abuser" is a circumstance (and hence a valid skos:Concept)
with some skos:narrower concepts.

So I suppose my question is about whether it would be useful or correct to
use skos:Collection in any case here.

Any thoughts welcome.

Thanks in advance
Mike