Clarification on 'relationship' element of choreography

Wondering the benefit of this element, as the relationships associated with a choreography could be inferred from the interactions in the choreography.

If the purpose is to restrict the relationships that the choreography can be involved in, then shouldn't this be optional? That way, if it was important to ensure the choreography only dealt with particular relationships, then they could be specified - otherwise in the absence of this element, any relationship can be used.

Second point about this element is, if a parent choreography has specified the relationship(s), then does this imply that the enclosed choreographies can only be associated with either the same, or a subset of those relationships?

Regards
Gary