Multi-Master is simply setting more than one server as “Grid Primary”. In general, you set one to “Grid Primary” and the others to “Grid Secondary”. If you set multiple to Grid Primary, any Grid Secondary will, at configuration or editing in the name server group, allow you to set the preferred primary via automatic selection or manual process.
If you query any Grid Primary for a multi-master zone, it will always return itself as the SOA. The secondaries will refer to a preferred Grid Primary but, if it is removed, they will start referring to the next Grid Primary.
There are performance implications of being multi-master. Consider multi-master only in scenarios where things are highly dispersed (e.g. across continents), where you have clients doing updates to a zone that spans the regions. No more than three or four members. Five or more multi-master nodes in a Grid raises performance issues. So, you can have eight DNS servers for a zone, but don't make every one a primary.
Also consider it where GM/GMC are the DNS servers and you want to allow for DNS updates when GM dies and GMC hasn't been promoted yet. Multimaster is okay in smaller scenarios.
Dynamic updates take priority over normal queries. High rate of dynamic updates will cause a stop to normal queries.
Example: If you have eight DC spread globally and each DC has a NIOS DNS server, do not make all of them Multi-master. Way too many. You should max it out at 3 or possibly 4. Remember, each primary has to feed its changes back up to the Grid Master. The GM has to then soft it out and track info and serial numbers for each primary and then redistribute it out. (Microsoft would just add it to LDAP for replication). This is all doubly true if Dynamic DNS is involved and even more so with GSS_TSIG.
NOTE: You cannot have multi-master enabled on a zone that has DNSSEC signing enabled.