Can and could uses

Hi,
I think I was looking at the Englishpage.com website for help on modal verb use and I think in the tutorial on it, I saw something like the modal "can" is usually used for a generalization and a supposition. Does that mean the modal "can" is limited in the possibility department than the modal "could" in the same function -- possibility?

Unlike "can" can "could" prevail for virtually all types of situations including situations involving suppositions and suppositions?