Heat treaded alloy fasteners are designed to be stretched to a certain point to retain their clamping load. It’s up to the design engineer to establish which size fastener, how many, and how tight.
Proof load: the stretching load allowed before deformation and or functional failure.
Tensile load: The ultimate stretching load allowed before the bolt strips or fails. The proof loads of hex nuts are designed to exceed the tensile load of the matching hex bolts in support of the theory that the nuts should be the stronger component.