SOC vs. DOD
Battery life and performance are often expressed as either a function of their "state of charge," or of their "depth of discharge." If these terms were applied to your car’s gas tank, the state of charge (SOC) would tell you how much gas is in your tank, while the depth of discharge (DOD) would tell you how much air is in your tank.
With regards to a battery, a fully charged battery is at a 100% SOC and a 0% DOD. If it has 75% of its charge, is at a 75% SOC and 25% DOD. Storage companies very often use these terms with relation to the warranty for their battery product, or in relation to its product Life Cycle assessment.
XP's warranty explains how the "percent change in state of charge" relates to the number of expected cycles for its storage medium, the PowerCell™. Given the services that an integrated storage system provides, it is not necessary to benchmark performance off a starting point of a fully charged battery because a battery will be required to be able to charge and discharge at any given moment. The battery’s charge may hover around the 50% DOD / 50% SOC, where it has the maximum ability to respond in either direction in response to the needs of the grid. Therefore, the distance of each up and down swing of the charge, or the percent change in SOC, is most relevant to the battery's expected life and performance.