2. Find the system action

The action of the system under study characterizes its essential raison d'être .

You've already listed all the actions the system needs to perform, starting from the user's needs; do it again with a fresh, candid eye. Instead of starting with the user's needs, start with the product.

You need to find all the system's service functions (SFs). If you find more than one, it's because you've forgotten a function or you've formulated the functions incorrectly (used the wrong verb or complement).

Example 1: the system studied is an armoured door