Product Backlog refinement is a continuous activity. When an item is at the top of the backlog,
which means it will be developed in the near future, more refinements are usually needed to make
sure it is clear and contains just enough details (not all possible details). It's preferred to have
the items that will be selected in the Sprint Backlog completely refined before Sprint Planning;
however, it's OK to have more refinement done during the development. Remember that there's no such
thing as Sprint 0 and there's no time between two Sprints that can be spent on something such as
refinement. The team is also cross-functional and does the a-z of the work needed for each item,
including refinement; they don't use people from other parts of the company.