ResourceGroups are mandatory in ERN

ERN-3

In ERN-3, all resources that are part of a release are referenced from the relevant Release composite in its ReleaseResourceReferenceList. This list is, however, only a "bill of material". It does not describe the order in which the resources are used to make up the release. This data is contained in the ResourceGroup. To ensure this is the case, it is mandatory to include descriptions of all resources into the ResourceGroup, together with the indicator communicating whether a resource is "primary" or a "secondary".

Therefore, the answer to the question is “yes” for ERN-3. This includes simple releases, such as singles.

ERN-4

For ERN-4 the situation is somewhat simpler. This is because ERN-4 does not have a ReleaseResourceReferenceList. As a consequence, the ResourceGroup is the only means, in ERN-4, by which the resources that make up the release can be listed and described. This composite also contains data which describes how the listed resources make up the release.

More on ResourceGroups can be found here as well as in the ERN-4 samples here.