This post should serve as a sort of myth-dispeller about action combat (BDO, New World, TERA) vs tab combat (World of Warcraft, FFXIV) in MMOs.
The short of it is that in action combat, hit validation is performed by comparing the area-of-effect of an ability to the position of relevant opponents. In tab-target combat, hit validation is performed by comparing the area-of-effect of an ability to the position of your target. That's it.
There's a lot of historical baggage associated with these two validation schemes, but at the end, this is all there is. Say, for example, you have an ability like frost nova in World of Warcraft (a tab target game). When you press frost nova, the game knows the radius of frost nova (10 yards), and then freezes everything within 10 yards of the mage. That's an "action" ability.
Similarly, in world of warcraft when a mage presses fire blast, it checks to see if the target is in range (40 yards), and then if it is, then they get fire blasted. That's a "tab target" ability.
Different tab target games handle their hit resolution differently. Some games may want you to also be facing your target. Some require that you have line-of-sight and introduce objects that block line of sight. Some have facing requirements (rogues can only backstab if they are facing their target, their target is within range, and their target is not facing them). Some have cast times, where you need to be within range and within line of sight at the beginning of the cast time and the end of the cast time. Some only require the beginning, some only require the end.
Likewise, action combat games also handle their hit resolution differently. Some games like Dark Souls create moving hitcubes and compare those to hurtcubes like how combat works in a 2d fighter, but in 3d. Other games use simpler distance measuring (like if all of your skills were frost nova).
Separate from these ideas, for historical or traditional reasons, these games also tend to emphasize different skills. I wrote a pretty long writeup about what difficulty actually is, but the TLDR is that when we interact with our mouse and keyboard difficulty only comes in three forms: figuring out the right button/mouse motion to press, pressing complex sequences correctly, and pressing those buttons/sequences with varying degrees of timing leniency.
Tab Target games tend to focus almost entirely on the "which button is the right button" part. The game tends to give you a bunch of information (buffs, cooldowns, debuffs, etc) and a bunch of different (say, 30+) abilities you could press, and make you puzzle out which one is the most important to press right now. Then, once you've chosen, you can't press another button for ~1.5+ seconds. They soften timing requirements by making it so that if you press a button early, it "buffers" the press and so that you can just spam the button you want to do next and not have to worry about timing it.
Action Combat games tend to have less buttons to worry about, and also make the puzzle about which button is the right button less difficult. They tend to have less buffs, cooldowns, debuffs, etc to keep track of. In exchange, they make you perform more sequences and make you care more about timing. For example, maybe if you're barely out of range, your attack will miss in an action combat game, whereas in a tab target game you just wouldn't be allowed to press the button yet. An action combat game might make it so that if two people attack each other at the same time, whichever one's attack comes out first interrupts the other (like a 2d fighting game). They might give players ways to avoid each other's attacks via some sort of dodge roll (like dark souls).
Important to keep in mind is that none of these traditions need to be carried on. Whether or not you have a target doesn't preclude dodge rolling or hit stun. Tab target games don't need to have an auto attack, even though many do.