In the implementation of something like a deque or merge sort, you could have a variable that represents offsets from pointers but which could sensibly be negative. C developers culturally aren't as particular about theoretical correctness of types as developers in some other languages - there's a lot of implicit casting being used - so you'll typically see an `int` used for this. If you do wish to bring some rigidity to your type system, you may argue that this value is distinct from a general integer which could be used for any arithmetic and definitely not just a pointer. So it should be a signed pointer difference.

Arrays aren't the best example, since they are inherently about linear, scalar offsets, but you might see a negative offset from the start of a (decayed) array in the implementation of an allocator with clobber canaries before and after the data.