In a fractional scaling setup you first create a homogeneous virtual screen at the dpi wanted then get the right size by scaling the monitors out to fit. A dpi aware application will draw itself at the correct size on the appropriate screen. The main difference is the dpi aware application still looks good on all monitors. where as the fractional scaled stuff suffers scaling artifacts.

Wayland only supports fractional scaling. and there is a good argument that this is a better system, not because it looks better, it does not. but because applications don't have to be aware of it where a mixed dpi requires the application to actively deal with it.