Ghost State
Represents an additional state, a player can be in, when teleported upon their revival. Whilst in this state, a player can't be harmed and is invisible, but can't engage in either PvP or PvE, without being forced out of this state. The last part is handled by the PlayerManagerEvents event-listener.
It Is visible because it's a part of a player's data, which is public. However, all its members are either private or internal.