We differentiate customers from stakeholders in Lean Six Sigma because customers are the focus of projects. Stakeholders are important to the project because we need their engagement in order to ensure lasting change.
For customers we develop specific efforts to understand “voice of the customer” and how to measure their requirements. Their requirements are the focus of projects and method for determining process capability. This differentiation is useful when conducting process improvement projects.
From a corporation’s standpoint suppliers, customers, employees and investors are all stakeholders, but for the DMAIC method we find the differentiation useful.