I'm not exactly sure what xDT means as opposed to -DT, which I suppose (since it's associated with ArP ammo) designates that it reduces DT, making it easier to pierce armor.

Does xDT mean that it increases DT, making it harder to pierce armor?

Yes that's exactly what it means. You're no doubt looking at some hollow point ammunition. Hollow points *triple* the target's effective armor, but, they do 75% more damage after armor reduction.

If you had a rifle that did 40 damage, and was shooting hollow points at a target with an armor rating of 5: the math would look like this
damage = max((40 - 5*3) * 1.75, (40 * 0.2) * 1.75)
damage = max((40 - 15) * 1.75, 8 * 1.75)
damage = max(25 * 1.75, 14)
damage = max(43.75, 14)
damage = 43.75

Whereas if you shot that same bullet at a target that had a DT of zero, it would do 70 damage.

So. Hollow points for targets with no armor, normal (or better, armor piercing) rounds for targets that have armor.

Aye, thanks for the mathematical breakdown, very helpful!