So p99 is an EqEmu server (or are 3 servers).
There are a bunch of other people who also host their own servers on eq emu, but those aren’t p99 servers.
Most of them are basically private playgrounds and sand boxes. A few of them are ‘practice/training servers’ of p99 guilds. They all have a bunch of different eras/content.
Many require their own specific patches.
A popular one, which I believe plans to run up to PoP is Quarm, for example, and actually uses an older client (i.e. older version of the everquest program the player runs on their computer).
I hope this explains things a little more clearly. The home page of p99 includes a sidebar with a link to eqemu
http://www.eqemulator.org/