subcards isn't intended to imply plus cards.  at present we have two different vocabularies:

children / parents = name relationship.  eg A+B is a child of A and B
subcard / supercard = nesting relationship.  if C includes D, then D is a subcard of C.

I may go through now and fix a related issue in the Format object.  At present we have subformats, which are a direct parallel to subcards.  If C includes (has subcard) D, then C's format object creates a subformat object to build/render D.  The part that needs to be fixed is that we currently refer to C's format object as D's "parent".  To be consistent, that should be "superformat".

Feel free to propose different nomenclature, but I do think we should embrace this formal distinction.  (And in any case, this is a clear improvement over just "cards" which is very ambiguous.  I'm not blaming you for that; I thought I was the one who created that bad name in the first place!)