Riverside County and San Bernardino County to the north form this metro area. Like many such metro areas in the Southwest, it extends far into uninhabited desert areas, in this case east through the Mojave Desert to the Nevada/Arizona border. Larger than nine U.S. states, it is often referred to as the Inland Empire.
I agree with tchrist that the word doctor in doctor appointment is a noun (attributive noun I believe), although the definition of an attributive noun is a noun used as an adjective, so maybe it's just semantics at some point.
Appointment letters are professional and business like documents which convey the expectations of the company which the employee is expected to fulfill. Hence, they must be written carefully by professionals who are well versed in framing such letters. Appointment letters must be concise and polite in tone.