We need to distinguish capacity and state of charge. State of charge (SoC) is the level of charge of an battery relative to its capacity.
Capacity is the quantity of electricity that a fully charged battery can deliver. State of charge is a characteristic giving information on the degree to which the capacity is filled.
Example
For example 100Ah battery can contain 100A*h if a battery is fully charged and as an example 50 A*h if the battery charge is not full. SoC is usually is measured as percentage 100% = full, and 0% = empty.
SoC is normally used when discussing the current state of a battery in use.