Software product lines are used by companies to provide a set of reusable software assets sharing common features that satisfy a market. Though a product line is based on the expression and the capitalization of afamily of similar systems, the differences between products must be identified and expressed in order to be able to build the applications by taking the good decisions. Today, even if the flexibility aspects are well covered, there isn’t as many studies on variability description and usage. This paper presents a process, methods and techniques investigated in THALES (Research Center and Business Units) to express the variability between products and its usage to derive new products from the Software product line.