Userid
A userid is a unique integer given to each player who connects to a server. Numbers are assigned sequentially, starting from 1, and never re-used.
IDs are chiefly used to identify players in events.
Warning:It can be easy to confuse a player's ID with their entity index, as until someone disconnects from the server they will be in sync.
Usage
IDs are assigned by the engine, and cannot be changed.
engine->GetPlayerUserId(edict_t*)
player_info_t::userid
IPlayerInfo::GetUserID()