with is recognized as one of the Extended RValue→ expansions. It introduces a new map.

... with
  %Item0% Extended-RValue0
  ...
  %ItemN% Extended-RValue0
end

Creates a map with the values specified (each value is subject to further expansion as an Extended-RValue). Depending on where the "with" is used determines what happens with the created map.

Standard item notation→ is used.