We pick m.root-servers.net, one of the 13 root servers (1), and ask it about ns5.digitalserver.org and learn that org is delegated to six name servers (2), so we ask b0.org.afilias-nst.org where we learn that digitalserver.org is delegated to two name servers, including ns5.digitalserver.org (3), so we ask that one and get the following answer:
The primary name server ns5.digitalserver.org only
All of the 13 domains that use the primary name server ns5.digitalserver.org use name server group 1. Worth noting is that all of the domains that use name server group 1 use the primary name server ns5.digitalserver.org.
The two name servers ns5 and ns6.digitalserver.org together
There are 13 domains that use name server group 1.
All of those use the primary name server ns5.digitalserver.org. Worth noting is that all of the domains that use the primary name server ns5.digitalserver.org use name server group 1.