Main Display and Overscan Issue

Maybe somone can explain what's going on here. I have a MBP 14 connected to a doc via usb c to 2 monitors and 1 LCD TV. The LCD TV always has overscan problems and cannot be adjusted in the TV menu.

Sometimes the underscan slider is available in the display setting on the mac, sometimes it isnt. But I just noticed this time the LCD TV is set to "main display" and the underscan slider is now there. Cool. I hope it remembers this setting.

My question is, what does "Main display" even mean? They all seem to act the same.

Also, this might help someone else who is missing the underscan slider.