An add-on represents a diversification of a business endeavor. When a busess grows vertically, it is said to be "adding onto its business." When a business grows horizontally, it is said to be "diversifying."

So when a carpet cleaning company adds more trucks and cleaners to its business, it is growing vertically. And when a carpet cleaning company adds water-damage and restoraton to its business, it is said to be "growing horizontally". It is the same with crime scene cleanup's horizontal and vertical growth.