Purpose of MDRS (Module Dependencies)

This month we encountered some problems with the MDRS (as a result of publishing a couple of new modules), which led to questions around the purpose of the Module Dependency Refset which has potentially evolved over the years.

So our scenario is we are now publishing an (optional) module within our release. The module contains a simple type reference set. The members of the simple type refset come from both core and the AU-extension. So we stated the following dependencies.

Optional Module → SNOMED CT-AU

Optional Module → SNOMED CT core

Optional Module → SNOMED CT model component

However, there's apparently an argument that the MDRS describes the the Edition dependencies. I don't agree with that, but it does raise the question how is an 'Edition defined' (if not by the MDRS)?
(Edition being a specific set of modules). This is probably a relatively new problem, as most editions (as far as I'm aware) only use a single module. So the module dependency as I describe earlier ALSO defines the edition. However when there's optional modules (like ours) these are two different things. The likely solution we're looking at is to have one module that represents our "Edition", and another that holds our "Extension content". To date Edition and Extension have been represented by the same moduleId.