Each time you zoom in, zoom out, or resize the map, the scale changes. The change is reported on the scale bar, which represents the ratio between a measurement on the map and measurement on the earth (for example, one inch = 40 miles).
The panel has two parts: selecting the data source units and setting the scale bar units. The data source units must reflect the units of the data in your service. For example, if your data is in meters, then the data source units should be set to meters. If this is not set correctly, you will find that distances and measurements on your map are incorrect. The scale bar units defines what the user sees on the screen. The left-hand side is the screen units such as one inch or one centimeter. The right-hand side is the earth units such as miles or kilometers.
