I am going through this course on collision detection: https://siggraphcontact.github.io/
In this link is a PDF called course notes. Scrolling down to section 1.3, called constraints.
In this section it is said that we can write bilateral constraints as ##\phi(\mathbf{x}) = 0## and unilateral constraints as ##\phi(\mathbf{x}) \ge 0##.
I understand that, but then it says that these constraints call also be written as:
$$\mathbf{J} \mathbf{u} = 0, \mathbf{J} \mathbf{u} \ge 0,$$
where...