In AGOL the measurement widget act differently depending on the cordinate system of the base map. In most cases we use a base map in Web Mercator (Auxiliary Sphere) (EPSG:3857) and the line type is geodesic. In other cases we use another projection for the base map and the line type is then planar (or atleast that seems to be the case).
It would be useful to display in the widget what distance (and area) is actually calculated, since currently this is a mystery for most users. Maybe also give the option to swith between different line types independent of the basemap. Since many users are not that interested in what is being used it can be a more hidden setting as well, configurable for the app/map designer.
