2 Band vs. 3 Band

Can I solicit some explanation of 2 and 3 band brushes? What makes them different and why don't I ever see these descriptors mentioned in retail ads?
Thanks for the help. It's been bothering me for a while.

Check out the below links from TSN vendor Wet Shaving Products.

http://wetshavingproducts.com/badger-hai...-a-primer/

http://wetshavingproducts.com/badger-brush-hair-grades/

Thanks Branford. Very informative.