Just quoting from the SETP paperwork (not contradicting anything DAR has said, just expanding)...
A. EXPERIMENTAL flight testing is defined as flight testing which investigates the characteristics of an aerospace vehicle or its components under conditions not previously tested. Examples include first flights, envelope expansion, and initial performance or flying qualities testing of new or significantly modified vehicles.
B.DEVELOPMENTAL flight testing is defined as flight testing which conducts the initial investigation of the effects of any engineering
or design change to an aerospace vehicle or its components. Examples include structural changes, control law development, and certain systems tests. For systems tests to qualify as developmental, the tests must be of systems under development that are used by the pilot to assist in the control of the vehicle
And to quote Rusty Lowry, recently of USAFTPS and a good and knowledgeable chap...
The modern test pilot is an engineer in a flight suit.
G