There are blocks with category or we can say Types, Examples are dirts/sands/planks/wool/stone/clay and when lets say, we create a custom plank
An option to make the block go into Planks category so it can be used as plank for all crafts that requires wood planks, It also depends on Other mods which uses planks in their recipes so we can use our Custom plank in their recipe
same thing for saplings(still waiting for trees improvements)
Also for item type, custom items that can replace vanilla items, like dyes, leather etc...
i think you know what i mean
Oh and for stone, Stone is a category too, His category is Diorite, Andesite, Granite, which means that These blocks will always spawn with stone(sometime)