Combat / Targeting
The yellow lines displayed when an enemy is selected are referred to as "aggro lines." Enemies linked by aggro lines will attack en masse when one of their number is attacked.
Aggro lines are displayed if there is a significant difference between your character's level and the enemy's level, or if you are accompanied by several other party members. Please be aware that aggro lines can also appear when summoning your chocobo to fight alongside you, so be sure to assess the battlefield before stepping into the fray.