Unit System shows the unit system for the active document in the status bar and lets you change or customize the unit system.
Unit System is located next to Quick Tips in the status bar.

For example, MMGS is displayed when the unit system for the active document is millimeters, grams, seconds.
To view the full unit system name in a tooltip, hover over Unit System with the pointer.
To select a different unit system or to customize the unit system, click Unit System, then select a unit system, or click Edit Document Units to open the Document Properties - Units dialog box.