The altitude Status widget shows you a summary on the actual altitudes of the UAVs in the swarm:
The top row shows the maximum altitude.
The bottom row shows the minimum altitude.
UAVs can measure altitude relative to ground (AGL), relative to mean sea level (AMSL) or in a local Z coordinate. If you move the cursor over the widget, its tooltip will tell you which altitude source gets displayed actually and which will be displayed if you press the widget icon.