To Build lets you set the quantity To Build for components in a kit. This was requested by a drink distributor to solve the problem of stocking and selling cases and sometimes selling singles. Selling one single from a case leaves 23 singles, not 23/24ths of a case.
This is solved by creating an item for singles and setting its Kit to use 1 of the case item To Build 24 of the single item. Big Business automatically converts full cases using Auto Build and updates Cost, Landed Cost, and Ship Weight for single items.