So, because I didn't really know how this newer targeting system works I thought I'd sit down and figure it out. To be clear, this game has *three* targeting systems:
<b> - Per spell targeting
This system activates when attempting to cast a targetable spell (like a Heal spell) without having a target pre-selected by either of the other systems. You select your spell, you select your target, you cast your spell. Wash, rinse, repeat.
This system's targeting box is a deep BLUE color.
<b> - Old Tab Targeting
Default setting: "v" key
This system is activated by pressing the "v" key and then using the arrow keys/mouse to indicate which target you wish to select. Then you press the "v" key again to "lock on" to that target. You can now cast spells and bypass the "per spell" targeting system
This system's targeting box is also a deep BLUE color.
<b> - New Tab Targeting
Default setting: "tab" key
This system is activated by pressing the "tab" key, and then repeatedly pressing that key to cycle through targets. Once your desired target is selected you can cast freely as though it were "locked" as per the "old tab" targeting system. You do not need to press "tab" again to "lock on".
This system is a bit more complicated, because it has two modes by which it allows you to cycle through targets. The first is a TEAL-colored box which allows you to cycle through player targets. The second is a RED-colored box which allows you to cycle through non-player targets (NPCs/monsters).
You can switch back and forth between these two systems holding down the "control" key and pressing "tab" once. You should then let up the "control" key as holding it down further will only cause you to swap back and forth between players and non-players as you try to cycle with "tab" key.
Finally, by holding shift and pressing tab, you can cycle backwards through targets. Just tab cycles right then down, holding shift causes tab to cycle left then up. Credit to Tyania for informing me of this.
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It should be noted that the OLD tab targeting and NEW tab targeting can be swapped so that the OLD tab targeting is on TAB and the NEW tab targeting is on "v". While it's a very useful configuration setting, it does make it a bit difficult to differentiate between the two systems in this guide.