Underscan

Underscan makes the picture smaller than the available display area, often leaving borders. Overscan does the opposite: it enlarges the picture and crops its edges. You may encounter these terms when a TV or projector does not fit the Mac's image correctly. Apple explains overscan and underscan

BetterDisplay has two adjustments with the Underscan label. The native control under Image Adjustments changes the system-supported display setting. The slider under Stream Display scales that stream's presented image, with 100% as the neutral setting.

The stream setting does not reprogram the target display's native setting. Neither adjustment creates a new resolution or guarantees that the TV stops applying its own overscan. Choose the adjustment according to which part of the image path you intend to change.