Safe area
Also said as Title safe, Action safe, Overscan
The part of the picture you can trust a screen to show. Many TVs blow the picture up slightly and crop its edges, which is called overscan, so anything important stays inside a margin. The broadcast standard keeps titles within the middle 90%.
Without it, a HUD can be cut off on someone's TV and you will never see it on your own monitor. Phones have the same problem with notches and rounded corners.
Say it
“Keep every HUD element inside the middle 90% of the screen, and add a safe-area slider to the first-run settings so people can pull it in further.”
Where you’ve seen it
PlayStation consoles have a system setting, Display Area, where you drag the picture's corners until they fit your TV, because the console can't tell by itself how much it crops.
Not to be confused with ui scale
UI scale changes how big things are. The safe area changes where they are allowed to sit.