Design File Settings are where you can set up the file environment that include dimension units, resolution, grid & angle locks, scale & snap settings, etc.In the previous version (V8i), you could access from Settings > Design File, but now in CONNECT Edition, this is shifted in Backstage.
To access the Design File Settings while your current model is running, you need to go to Backstage, File > Settings > File and then need to go to Design File Settings.
In the Design File Settings dialog box, you can change settings like Active Angle, Active Scale, Axis, Color, Grid, Working Units, etc. like in the previous version.