The Application class uses it to calculate the application's scaleX, scaleY, x, y, width, and height.
It looks like I haven't added a proper description of this one to the API reference yet. It's not 100% the same as ScreenDensityScaleFactorManager in the Starling version, but it's basically the same concept. So you might want to read through those docs to get an idea of how it calculates the scale. Just understand that how the scale gets applied is a bit different. In Starling, it adjusted the stage and view port dimensions. In OpenFL, the application's transformations are adjusted instead.
To get the current scale at runtime from anywhere, you can use Application.topLevelApplication.scaleFactor.