After all, everyone's disbursements are someone else's income. It's just common sense. A represents payments into account balances held by consumers.
B represents payments into account balances held by firms.
T2 minus T3 at TX is the rate of instantaneously measured accumulation to entrepreneurial profit. Within the structure of production every B payment is a payment made to a lower stage of production, creating a cost passed up the structure to the point of final consumption. The pool of funds into which B payments are deposited does not constitute effective demand against final production. Nor does it need to, assuming steady-state.