Who is the customer in organizational hierarchy?

In a business organization hierarchy, where E (engineer) reports to M (manager), who is the customer in the E - M relationship?

Our first inclination may be to think M is the customer -- M is the boss, E should make M happy. But, lets think about this a bit more. Who does E create business value for? Is E writing code that benefits M, or producing a printed circuit board that M will directly use?

Hmm, does not seem so. E's customer is probably purchasing and manufacturing (in the case of a PCB). M does not directly produce tangible outputs. M's job is to help E get the job done. Therefore, it would follow that E is M's customer. Certainly, M has authority and needs to make decisions at times. But, in the end, the job must get done. Business value must be produced. In product development, authority or decision-making in themselves do not produce business value. The more people we have under us, the more people we serve.

Do we exist to serve, or be served? The higher up we go, the more important it is to understand servant-leadership.