Focus target and acquire focus targets target
These two go together focus target has some uses without the other but most of the time if used in unison they work great together. They can also be moved around and resized in the interface editor.
Focus Target -
In Preferences under bindings and targeting there is a set focus target / swap focus target entry. You can keybind a key or set of keystrokes so that when you click on someone and then hit that key or keys a focus target frame will come up.
Acquire focus target target's -
Same place as above just a little further down on the list. This keybind will allow you to acquire the target of your focus target once its set.
One is to focus target the boss then if there are adds you can keep an eye on the casts from the boss while taking down the adds.
It can also be good if your new to ops and are asked to follow another dps. Make the dps your following your focus target then hit acquire focus targets target keybind to make sure your on the right enemy.
Healers will also sometimes focus target a tank so they can keep one eye on the tank during high enemy dps.
This is where these keybindings tend to shine.
Tank - One of the major roles of a tank especially in 4 vs 4 is to swap guard and fast depending on who is being targeted. Setting you focus target as the others teams top DPS and then whenever needed you can acquire heir target (your teammate) and have a good guess on who they are switching to and who to guard. And since it is fast (just one key press) and they are already targeted you can place a guard on them quickly.
DPS - two good was to use this in pvp as a dps.
1) everyone focus on the same person and swap at the same time. ever wonder how good teams swap and focus so well. They designate one dps to look for the guard from the tank, everyone else sets their teammate the lead dps as their focus target. Now anytime the lead dps swaps (even better ion voice all they have to do is say swap) everyone else just hits their keybind to acquire their focus targets target and your all dpsing the same per quick.
2) stuns and interrupts. If their is an opposing healer, (Or sometimes a dps) that you want to keep stunned and interrupted. Place them as you focus target that way while dpsing everything else you can make a quick swap to them to interrupt or stun.
Same as above for DPS - allows you to quickly stun or interrupt a focused opposing player while not missing a beat healing.