Major The current $batch specification says that requests within a changeset need not be processed in the same order they are specified. For example, a service may re-order changes to reduce the likelihood of integrity violations by first doing deletes, then inserts, then updates. It goes on to say that the responses in a changeset must be 1:1 with the requests, but does not say they must be in the same order, and does not say how a client correlates a response within a changeset with a particular request within that changeset.
Changesets do describe the use of a content-id header for allowing a statement within a batch to reference the results of a previous statement within the batch. For example, to add an order to a customer that was added within the same batch. However, it does not say that it is mandatory for the client to specify the content-id header for requests within a batch, nor does it say that it is mandatory for a service to return content-id headers supplied within the batch.
It seems simplest to say that clients SHOULD specify a content-id header for requests within a changeset, and that service MUST return the content-id (if any) specified by the client. This allows the service to re-order changes within the changeset without having to buffer the response, and is arguably simpler for the client to correlate responses without having to rely on ordering.