Interesting question. Off the top of my head I would say the order is determined by whatever the query returns. AFAIR the query will return all the modules on that page and that union to add modules that are marked as on all pages. If you wanted a particular order you could probably swap the data around in the database. However most people pass information in other ways i.e. via the querystring, or add it to a request level item (personally I use request.items but others use cookies etc), or add it to a page level item such as v13wstat3. Alternatively you can use the inter module communication (IMC) support to get modules on a page to communicate with each other.

Cathal