"Bullseye" is a term used in military radio communication. The bullseye is a pre-defined point which is used as a reference for radio calls indicating a position. For example, if someone says they have a radar contact at "bullseye 050 at 30 miles", the radar contact is 30 miles away from the bullseye point at a heading of 050 (roughly northeast).
Bullseye is used in case radio communications are intercepted by the enemy - since the bullseye point is agreed upon prior to the flight and is not stated over the radio, the enemy will not be able to determine where you are referring to if you use bullseye terminology. In addition, using bullseye coordinates helps improve situational awareness, since it is an absolute frame of reference. If AWACS tells another aircraft that they have enemy fighters at "7 o'clock, 15 miles", you have no idea where they are. However, saying that the fighters are at "bullseye 077 at 15 miles" tells you exactly where they are located.
Use of bullseye in radio communications can be controlled in the game settings menu. If active, bullseye will be used, and if not, standard bearing/range will be used in radio communications. You can view the location of the bullseye point in the theater map by right clicking on the map and selecting "bullseye"
The bullseye is displayed on both the FCR and HSD as a bullseye symbol. In addition, the navigation cue on the lower left hand side of each MFD page can be changed to a bullseye cue. To do this, press the LIST button on the ICP, then select MISC, and then BULL. Pressing the 0 key on this page will toggle between the bullseye cue and the regular course deviation cue.
The bullseye cue displays three important pieces of information:
The relative bearing to the bullseye is displayed by the triangle on the outside of the circle. If the triangle is at the top of the circle, your aircraft is pointed at the bullseye. Your aircraft's current bullseye position. In the picture below, the aircraft is at bullseye 211 at 5 miles. The bullseye position of your cursor. In the picture above, the cursor of the HSD, the lower right bullsey, is at bullseye 049 at 9 miles. This is useful for identifying places to go to, or look out for. The same place on lower left side is the FCRs current targeting cursor, shown on HSD. It is currently pointed at Bulls 347 for 17 miles. It is useful for identifying friendly and hostile aircraft (for example, if you have an unknown contact, you would say "ray gun bullseye 347 for 17 miles", and if it is a friendly target, they would reply "buddy spike" when they look at their own bullsey after hearing "raygun" call).