Basically, any guild on the server that kills a raid target will fall into one of two categories:
Class R
Class C
Class C has few restrictions. There are no mob lockouts, and as a Class C guild you are free to engage and kill as many Class C and FFA mobs as you'd like. You can''t touch Class R spawns (You'll know its a class R spawn by viewing the page at
www.project1999.com/raid.php)
Class R guilds will receive a two
class spawn lockout on any mob they kill. So if you kill CT, you cannot engage the next two Class R CT's, but you can engage the next FFA CT. You could also engage a Class C CT, but that would move you to Class C.
The staff doesn't specifically enforce a rotation among Class R. There are a list of rules on the bottom of the raid.php page that further details the terms for raiding here on Project 1999.