Exhaustion refers to allocation. For example, my company has several very large subnets (we're a UK ISP), that are entirely unused, as we bought them by the mil

Exhaustion refers to allocation. For example, my company has several very large subnets (we're a UK ISP), that are entirely unused, as we bought them by the million when they were cheap back in the 90s. We could sell smaller subnets (we often do...), or we could use them for things like VPS hosting. Digital Ocean will own several blocks/subnets, and allocate from those pools.