The basic model for an electrical system in a building is a tree. The utility is the root, and the system branches out to panels and subpanels from there. Most devices are connected to a single source—either another device in the system, or the utility.
There are three devices that do not obey this rule: transfer switches, UPS’s, and generators.
Generators have no source at all. They are not connected to the utility or another device in the system. They are their own source of power and provide power to the devices connected to them.
Transfer switches and UPS’s have two sources. Typically, the first source is connected to a device that eventually leads to the utility. For transfer switches, the second source is connected to a generator, or to a device that eventually leads to a generator. For UPS’s, the second source is often not connected to anything, because the battery inside the UPS is the secondary source of power.
Design Master Electrical calculates fault and voltage drop from both sources. The larger of the two values is the one that is displayed.