Wisp targets all guards in the facility which the agency currently has vision on, including civilians and drones, even if they are knocked-out or rebooting. Wisp does not target guards on unrevealed tiles, guards in the “fog of war” (even if a “ghost” of the guard is visible), guards who are already TAGged, or drones that are currently controlled by the agency. If there are no valid targets, then Wisp cannot be used, and instead the message “all visible guards have TAGs” is shown.
Guards TAGged by Wisp remain TAGged even if Wisp is sold during the mission.
Wisp is considered to be one of the best programs in the game. TAGging is very powerful, especially if you can TAG every guard in the facility.
Wisp TAGs all visible guards who are not already TAGged, so to use it efficiently you will want to see as many guards as possible when you use it. While exploring the facility, you can see more guards at the same time if you split your agents up to explore in separate directions, and an agent can peek through doors or around corners to see guards in more than one area. However, even if you can only see 2 or 3 guards at once, using Wisp to TAG them is still usually advisable.
It is usually not necessary to have both Wisp and a TAG Pistol; if you have a choice between them, it will depend on your loadout and strategy. Wisp can be the better option when you have enough PWR to use it frequently, when you aren't short of Incognita program slots, or when you want to save your attacks for something else.