Suspect -> Prospect -> Customer -> Client -> Advocate
What's the difference between a customer and a client?
According to WiseGeek, "the main difference between a customer and a client is that a protective, ongoing business relationship is formed with a client, but not necessarily with a customer."
Blue-Ferret provides a similar definition: "Clients are relationship-based. They are long-term associations. Effort must be expended to cultivate a client relationship. Customers are transaction-based. They are transient, mercurial associations. Either party can and often does sever the relationship suddenly and at convenience."
Finally, BNet corroborates that "Webster's (1985) definitions ... Client has been defined as "one that is under the protection of another: Dependent" or "a person who engages the professional advice or services of another (lawyer)" or "a person served by or utilizing the services of a social agency (welfare)" (p. 248). In contrast, customer is defined as "one that purchases a commodity or service" (p. 318)."