A represents the money issued into circulation and B represents the prices demanded for the goods. In Douglas's notation, A are payments firms make to directly final consumers in the form of salaries, wages and dividends.
B are payments firms make to other firms or equivalent accounting entities, including government.
There are accumulating account balances in both the firms and consumer sectors. So even if consumers spend all that they receive as soon as they receive it, it would be insufficient to purchase the totality of increasing production. The argument is very much cast in the form of a reducio ad absurdum because consumers never spend all that they receive as soon as they receive it. Indeed, it would be impossible for them to do so. What it means is that entrepreneurial investment, in the statistical or macroeconomic sense for the economy as a whole, cannot derive from income diverted from consumption by the recipients of income, or saving, or redistribution through taxation, to the extent that such income is costed into production.