What is a Customer?
The customer is whoever will approve and manage the project's product, service, or result — and often the one paying. Distinguish the neighbors: the sponsor champions and funds the project inside the performing organization; the customer receives and accepts its output; end users actually operate the product day to day. One party can wear several hats, but the hats are different.
The distinction earns money at acceptance time: validate scope happens with the customer, whose acceptance criteria — not the sponsor's enthusiasm, not the users' delight — are what close the project.
Worked example
A construction firm (performing organization) builds a clinic for a healthcare group. The group's CFO sponsors and funds it; its facilities director is the customer who reviews and formally accepts the building; nurses and physicians are the end users. When the nurses request layout changes at month 8, the request routes through the customer and change control — sympathy is free, but acceptance authority has an org chart.