Hammer Tutorial V2 Series #12 “Input and Output overview”

The following special targetnames can be used to dynamically select an entity.

 Note:Not all entity parameters can evaluate these targetnames.
 Note:When used in an output’s parameter override field, targetnames are evaluated by the entity that receives the output, not the one that sends it. This can change what is pointed to.
!activator
The entity that began the current I/O chain. If a player walks into a trigger that fires a logic_relay, the player is the !activator of the relay’s output(s).
!caller
The previous entity in the current I/O chain. If a player walks into a trigger that that fires a logic_relay, the trigger is the !caller of the relay’s output(s).
!self
The entity from which the current output originates. If a player walks into a trigger that that fires a logic_relay, the relay is the !self of its output(s).
!player

The player. Only useful in singleplayer.

 Tip:To target all players in a server, use player.
!pvsplayer
The first player found in the entity’s Potential Visibility Set. The PVS used is taken from the entity doing the searching, or the activator if no searching entity exists. If no activator exists either, the first player in the game is returned (i.e. !player).
!speechtarget
The entity at which the !caller is looking due to a Look At Actor or Face Actor choreography event.
!picker
The first solid entity under the player’s crosshair. Only useful in single-player, and mostly only for debugging. Non-solid entities cannot be selected since the invisible “bullet” that is fired does not hit them.