Sure, but how do you prove that you won't end up with `x' + y' > Z` for some value of x' and y'? In practice, it's always easier to prove `x < X, y < Y`.

Generally the idea is that all of these are (more or less) easy to prove but the other policy allows for x' and y' to be independent

Dynamic stacks with linked granular chunks? It consumes even less memory.

Not contiguous

Obviously chunk size is larger than class of objects to allocate.

This is not always possible

Then fixed stack also is not possible for such cases.

Sure it is? Stacks are contiguous

If you can't limit stuck chunks you can't limit fixed stack. And vice versa. It's quite simple.