Not a philosopher by any stretch, but if I can decipher my Googlings of Heidegger’s interpretations of Descartes correctly:
The res corporea “the bodily substance” is defined in opposition to the res cogitans “the thinking substance”. Applied to people, it is the human body as opposed to the mind. The defining attribute of the res corporea is its extensio: the fact that it has physical dimensions, and thus, physical existence—something it shares with all physically existing things, but not with the mind, the soul, or God.
So the res corporea “human bodies” are a subset of all res extensa “things that have physical dimensions”. Descartes uses res corporea and res extensa interchangeably, since he is defining mind vs body. Heidegger uses res extensa separately from res corporea, as he’s defining not just mind vs body, but the physical world (which consists of more than just human bodies).