RPC is the acronym for Roleplay Character. An RPC is a major character controlled by a Roleplayer, and is used to do major actions, such as fight, negotiate, build, lead, follow, whatever. RPCs have more privileges than named NPCs. E.g., is an RPC and NPC were to get into a fight, the RPC would likely come out on top.
Though it has changed since the roleplay's beginning, the current limit of RPCs per Roleplayer is ten RPCs.