L3 is more of a lock on, The feature i am talking about is Focus target. Which brings up an additional Hp bar/cast bar/and debuffs of a target that you want to still focus on but are not directly targeting anymore. This is useful on the fights i talked about above. Say your in a Odin fate and Odin casts gungir, so you take your target off Odin to DPS down the gungir, now you can no longer see odins hp bar or the spells he is castings except for the red circles on the ground which are not helpful in such a large scale battle. Now with Focus target enabled on Odin it will bring up a mini HP bar of Odin but more importantly you can see the spells he is casting or readying while you are targeting a different target (Gungir). This helps ALOT when he is readying his petrify skill as that usually leads to your death when it connects, so with focus target enabled you can see him casting that spell while DPSing gungir and run away easily instead of being oblivious to it .