These structs together comprise a "normalized" TOSCA-compatible topology, which is a flat, serializable data structure.
They are meant to be generic and independent of TOSCA grammar, especially any specific version of the TOSCA grammar.
Though they are usually created by a TOSCA parser as its final result, it's entirely possible to write code that uses a different approach, for example by translating from a non-TOSCA topology descriptor into the normal structures.