The is headcount that is present and any given moment of time, and is always derived from some other previous stock and flows accrued to that initial stock over a period of time. There are two basic types of stock – initial and resulting. The latter is being calculated by adding the values of flow, taking into account their impact on the stock, to the initial stock.
The following diagram depicts the relationships between stock, flow components of both inflow and outflow kinds (upper and bottom areas, respectively).
Available aggregation levels