When you mark a step as ‘required’, visitors need to go through the required step before they can proceed in the funnel. If you mark all steps as ‘required’, you force visitors to go through all the steps of a funnel. When you only mark the first step as ‘required’, visitors will only enter the funnel when they match this step. When you only mark the last step as ‘required’, visitors will only complete the funnel when they match this last step.

It is common that only the first step is required, meaning users only enter the funnel when they pass the first step, and then may complete any of the other steps in a random order.