Two ways of describing the exact same battery level — one of which happens to matter a lot more for how long the battery lasts.
State of charge (SOC) and depth of discharge (DOD) are both percentages that describe how much energy is sitting in a battery right now. They are mathematically complementary — DOD is roughly 100% − SOC — so at any instant they're just two labels for the same physical fill level. But that equivalence hides something that actually matters in system design: how deep a battery is routinely cycled has a direct, measurable effect on how many years it lasts.