Let's say you have two categories numbered 5 and 11. These categories are no longer going to be used so you want to create a new category, number 99, to maintain the productivity for these now unused categories. When renumbering, you would specify categories 5 and 11 in the Old Number fields and category 99 in the New Number fields. The productivity information for categories 5 and 11 would be combined for the new category 99.