The IG describes how to manage subscription topics using an R4B interface, and how to send notifications using an R4 bundle, but the first resource in the R4B bundle is a subscription status resource. This means that in practice, the notifications are also R4B. However for both technical and policy reasons, I believe that implementers will want to send notifications using pure R4 (even if the subscription topic interface is R4B). To do this, the subscriptionstatus resource would be replaced with a parameters resource with the same kind of values as the subscription status resource.

This way, the notifications would be R4 and not raise any version issues for people who are implementing an otherwise R4 interface (the notion is that the subscription topic interface would run on a different end point that's R4B, and this is inherently more separable from the R4 core of the rest)