Additional Hidden balances come in sets. The balance number is a four digit number and is found laser engraved on the bottom of the balance near the balance disk. Hidden balances are only for use on Double/Single hung windows and additional balances should only be needed if the original balances that come with the original order are damaged/broken/lost.
Kerfing/Routing Jigs are jigs that attach to your handheld trim router and your plunge router. These jigs make it easier to route channels that are smooth and symmetric when working with wood windows. These jigs are recommend but are optional and not needed for the successful install.