Surprisingly, Caterus gives an accurate description or defintion of the formal distinction, though I am not sure what "objective" is doing there. Perhaps reinforcing the "real" aspect of the distinction holding between formalities that are one in re. As for Descartes, while I think he may be right to reject a formal distinction here, he has no idea what it actually is. I don't know what he means by "modal", for the modal distinction is not the same as a formal distinction or at least they are not synonymous; the modal distinction may be a weak grade of the formal distinction, but as usual this is highly disputed in the scholarship. I doesn't come up much in Soctus's actual writings. The place I've seen it is between the concept of being and its intrinsic modes, but there the intrinsic modes (if they're somehow related to a modal distinction) serve to differentiate finite from infinite being, that is, from two things that would be really distinct. But the formal distinction always holds between formalities that are one in re. Anyway, here are the texts, enjoy. I've copied them out of the Great Books edition.
End of the first objection, by Caterus.