Major Section 3 currently states that an entity can be a member of at most one entity set. Given the ability to model entity type hierarchies this restriction seems rather unnatural: "Manager" is a specialization of "Employee", so it seems natural to have two entity sets "Managers" and "Employees", the former being a true subset of the latter, and a new "Manager" entity, POSTed to either set, would automatically appear in both.
Currently the only way to do that is to have an "Employees" entity set and a "Managers" function, with the drawback that the function is not advertised in the service document and thus not consumable in a hypermedia fashion.