There are various types of press in a game of Diplomacy.
White press refers to messages sent to other players which indicates who it is from.
Grey press is sent out anonymously and black press refers to messages that are sent from one person but forged to appear as though they came from another person. The adjudicator provides the mechanism to set a game as allowing either or both of white or grey press. The press messages can be sent to individual players or can be broadcast to all players, depending upon the parameters set for the game. Black press is really the same as grey press with another player's signature in the message, so no distinction is made by the adjudicator, however, press sent to individual people can be faked to appear as a broadcast message to all people or to an alternate set of people if the particular game allows such a thing. The types of press allowed in a particular game are shown by the list command.