Pt = point

We use small artboards while the devices we design for are huge. For example:

• The iPhone XR has a Retina display that has twice as many pixels per inch (ppi) as a regular screen, meaning the artboard is rendered at twice as many pixels.
• On the iPhone X, the artboard is rendered at 3 times as much, since it has a Super Retina display and 3 times as much ppi.

Designing for the smallest size allows scaling assets into the various sizes the different devices require while maintaining pixel-perfect rendering. 1 pt can be translated into 1,4 or 9 pixels at the @1x, @2x, and @3x sizes.