Requirements Doc: Interaction among choreographies

 
In the requirements doc, we do talk about choreography and also
choreographies. It seems to me we need to clarify what this means? What do
we mean by interaction among choreographies. Do we need to define an
additional abstarct layer (language or whatever) that defines how two or
more choreographies will interact.
 
Can we talk about it in terms of set theory (Union of choreographies). Can
one choreography be formed as the union of many sub-choreographies?
 
If I am off on a tangent that is ok.
 
Abbie