The numbers are references to the size of the reel. To the best of my knowledge there is a vague similarity between brands with these numbers. Many manufacturers use the same reel between sizes with just a subtle difference i.e., a 2000 size reel from one brand is just a 1000 with a slightly larger diameter spool, but a 2500 model of the same reel will actually be a bit larger reel in general. The only constant is that smaller numbers mean smaller reels, perhaps with the exception of Mitchell, their reels get smaller as the number gets bigger, I think, I may be wrong though. All of that to say, use the reel you like on the rod you like. You will learn what balances well and feels right in your hand, and you will see a pattern form in the size reels you buy for different types of fishing.