Identify the variable and common parts of a line of products.
Invest in a platform of reusable assets.
Recoup the investment by developing further products with reuse.

Reuse can lower development costs and shorten development
   time with the same product quality, or better.
Reuse is the justification for using the product line approach

ConIPF defines a product line product development process that
   selects and configures available product components
It incorporates the results of a survey of the development and reuse
   practices of 5 large industrial organisations.
It accounts for product line evolution – when new components
   must be developed to meet new requirements.