Supported Aspect Ratios

In the player settings there is a section that allows devs to uncheck certain resolutions.
4x3, 5x4, 16x9 ect.

So what happens if I force my game to only be in 4x3 but the computer I run the application on only has 16x9 resolutions? Does it letterbox it or just not let the game run?

Thanks in advance!

So, the PlayerSettings->Supported Aspect Ratios is used to work our which resolutions to display in the resolution selection screen that the standalone can show when the game starts up. If you use supported aspect ratios, and this prevents any resolutions from being available, then rather than display nothing, or crashing, the list will show the current desktop resolution. This may mean that your game starts up in an aspect ratio that it’s not expecting. You should therefore check the actual screen resolution using Screen.currentResolution and do the letter-boxing yourself perhaps using Camera.pixelRect().