Protocol spec (2013-03-12) states:

10.4.2.2 Create Related Entities When Creating an Entity
A service that supports creating entities SHOULD support creating related entities as part of the same request.

We might wonder here (and in other sections in the spec, since I just gave this as one example) if instead of "SHOULD", we should see "MUST", in accordance with RFC 2119.

Use of "SHOULD" implies that there are certain scenarios where the server does not need to support creating related entities when creating an entity. If that is the case, then the spec ought to define a way for the server to advertise whether or not it provides this service, so that the client can formulate valid requests that are possible for the server to execute.