If the camera has Constrain Aspect Ratio enabled, then this should match Disabled, except the letterboxing will be cropped off.
ScaleToWidth
If the camera has Constrain Aspect Ratio enabled, then the width of the output resolution will be preserved, and the height will automatically be adjusted based on the camera aspect ratio.
ScaleToHeight
If the camera has Constrain Aspect Ratio enabled, then the height of the output resolution will be preserved, and the width will automatically be adjusted based on the camera aspect ratio.