A nexus point is the thing that anchors a realm of Suzerain to our version of reality. Change (or "flex" as it's commonly called) a nexus, and you could change reality. Flex it permanently and you change the prime reality and some lesser alternate reality takes over. Generally that's a really bad thing to mess with.
Because the various realms of Suzerain encompass all of mankind's past, present and future (as well as the realms of the gods and some alternate realities too), each realm is more than just a place. It's also a time, and may be something more abstract than that. Examples might help here:
American Grit is the name we've given to our realm set around an organization called the Special Projects Administration. It's the USA in the mid 1930s, where the SPA was founded. It's also 1999 and the whole world, because the SPA had operations going globally at that time. Being set before the Pulse War, it's a Timeline realm.
That was an easy one. Now for something marginally more challenging.
Untamed Empires is the name we've given our swashbuckling realm from the Elements group. This is the set of realms that exist for a couple of hundred years after the end of the Pulse War, before the barriers between them came down and mankind gets off the Earth. Although the trappings of the setting are very similar to the 1700s and 1800s, it's really a futuristic setting on the grand scale of things. In particular, Untamed Empires refers to a quarter of the Earth around the events 208 years after the War, so it's a point in space and time. Important things happen in the years after Yr208, and such important points in history are nexus points. Realms are often linked to particular nexus points - key events in history.
Okay, if that seems to make sense, let's try for the big tamale.
Circa Cycle is a realm based around an individual, a powerful spirit from the Maelstrom called Circa, who plotted to change history to become a god. Wherever he went, that's the realm. He hopped through space and time, sometimes in the mortal realms, sometimes in the Maelstrom among the god realms. But he was the nexus. He is the thing that may define history which makes him a realm. He surfaced in Shanghai in 2048 and led 'the good guys' a merry dance through many other realms as they tried to bring him down. Circa is a realm in his own right.