AR Measure uses the camera and the LiDAR scanner to measure openings in the real world.
What you need
A device with LiDAR: iPhone Pro models from the 12 Pro on, and iPad Pro from 2020. Without LiDAR the button does not appear. It asks for camera permission, and it needs a paid plan. An owner or admin can turn it off for the team under Settings, Team, Field tools.
Measure
Aim at the surface and wait for the status to read Ready. Tap the four corners of the opening to define a rectangle. Width and height show as you go, in your imperial or metric setting. Double-tap for the precision magnifier when a corner is on an edge. If a point will not place, move and try again.
Hold the device steady, 2 to 4 feet from the surface, away from direct sun. Mirrors and polished metal confuse it.
Accuracy
LiDAR readings are usually within a few percent at ordinary distances and worse far away, in bright sun or on shiny surfaces. Treat them as a quick field check. Confirm anything that is going to be ordered with a tape.