What is the purpose of the tank attributes "Use Low Alarm" and "Use High Alarm"?
If you set the "Use Low Alarm" to True you can enter a "Low Alarm" elevation (or level). If the water level drops below this value in a regular simulation you will get a warning notification, but the tank will still supply water (as long as it is above the minimum elevation/level).
If you set the "Use High Alarm" to True you can enter a "High Alarm" elevation (or level). If the water level rises above that in a regular simulation you will get a warning notification about it.
Whether you enter an elevation or a level is related to the operating range type for the tank. More information on operating range type can be found here: Understanding Tank Operating Range.
Note that the high and low alarm levels described above only apply to an EPS simulation. They do not apply to a transient simulation in HAMMER.
Note that you can add the high and low alarm level to a graph to visualize the change in water level and how close it gets to the alarm (or for how long it exceeds the alarm level). To do this, right click on the tank, choose "Graph", then in the Graph Series Options (right click in the graph and choose Graph Series Options if it does not appear), expand the Operating Range folder in the Fields section on the right and check the boxes for "Elevation (High Alarm)" and "Elevation (Low Alarm)":