Use this tag when developing apps that use the 3D Touch sensitivity feature on newer Apple Devices (iPhone 6s / 6s Plus and newer).
3D Touch is a new feature in Apple's iPhone 6s and 6s Plus that allows for depth sensitivity.
From the Apple iPhone 6s page:
3D Touch
The next generation of Multi‑Touch.
iPhone 6s introduces an entirely new way to interact with your phone. For the first time, iPhone senses how much pressure you apply to the display. In addition to familiar Multi‑Touch gestures like Tap, Swipe, and Pinch, 3D Touch introduces Peek and Pop. This brings a new dimension of functionality to the iPhone experience. And when you use 3D Touch, your iPhone responds with subtle taps. So not only will you see what a press can do — you’ll feel it.
Not to be confused with force-touch, which has one fewer sensitivity level.