From ADC

Flow describes enterprise's relationship with another entity. A flow is a simplest exchange of one resource, that the enterprise possesses, to another one, which it wants instead. Flows determine conditions, on which the enterprise accepts liabilities. It is assumed, that goods or services are completely interchangeable within each side of the exchange. The assumption is important for correct processing of the flow-related events in different accounting charts.

Storage flow is a special type of exchange, when the enterprises gives away and receives back the same resource. Except obvious storage examples for products or raw materials, most bank and stock broker accounts are also storage flows.