I found the picture shown below several years ago and as I'm currently studying about DNS and BIND, I wanted to understand and build the setup shown here. I do understand about the (aggregate) caching-only forwarders and it kind of makes sense to do the (internal) resolving yourself instead of just forwarding it on one more time to your ISP. The setup with the hidden masters and zone slaves also makes sense but I do not understand why you would put "external resolvers" in front of these zone slaves. Why not let external clients directly send queries to those zone slaves? What is the added benefit of inserting external resolvers?
I tried searching Google for this image in hopes of finding any document describing this setup but alas, I couldn't even find this image anymore on the web.