A product line extension strategy refers to a strategy that involves making additional versions of a product so that it will appeal to different clientele or making related products to sell to the same clientele.
For example, a company may make another version of a low-end product, which is a little better, and then make another version of it that represents the top of the line to appeal to different clientele. This is a strategy that allows a firm to take one product and extend it into several products without incurring significant additional development expense.