Clarification of the meaning of "unaggregated variable"

Hi,
I’ve been reading through the definition of SPARQL in the SPARQL 1.1. Query Language Document.  In particular, section 18.2.4, which describes how to convert aggregate queries into the SPARQL algebra.  
The algorithm listed in 18.2.4.1, which I’ve pasted in below talks about “unaggregated variables”.  However, there isn’t a definition for “unaggregated variable” in the spec, and it isn’t totally clear to me what an “unaggregated variable” is.  If anyone could provide me with a precise definition for what an “unaggregated variable” is, and thus clarify this part of the spec, I would really appreciate it.
Thanks a lot,
Matthew
For each (X AS Var) in SELECT, each HAVING(X), and each ORDER BY X in Q
  For each unaggregated variable V in X
      Replace V with Sample(V)
      End
  For each aggregate R(args ; scalarvals) now in X
      # note scalarvals may be omitted, then it's equivalent to the empty set
      Ai := Aggregation(args, R, scalarvals, G)
      Replace R(...) with aggi in Q
      i := i + 1
      End
  End