Chapter 21 closes Capital II. It develops by complicating the discussion of simple reproduction given in the preceding chapter. Here Marx insists that it is the technical composition of capital that determines the expansion of production, whereas simple reproduction is only notable for its demonstration of the circuits of capital’s value composition(s). There are some interesting comments on the consumption by workers, which speak directly the present tendency to get works to work more by encouraging them to buy more stuff: Marx comments on this directly (591-2).
The most significant point in this chapter is to do with what happens with that part of revenue that is capitalised, rather than hoarded or consumed.
It is a banal point that the distinction between simple (Chapter 20) and expanded reproduction is that a sum of value is capitalised. Marx does insist twice that this is the necessary condition of any real accumulation. But the important thing Marx introduces is that the total social capital may retain the same total value, but its use-value composition will change.