ff16, #148,
in my days (a long time ago), after the initial exploratory test flying, subsequent flights were jointly crewed with tps and training captains. This involved systems evaluation, normal and emergency procedures and handling, performance, cg range, auto-flight, instruments, etc. Some flights would have been to collect data specific to the design and manufacture of training simulators.
Later, training captains could captain production test flights depending on experience and the aircraft’s certification status.
In addition to CAA tp validation tests, operations inspectors and training staff flew on flights to evaluate workload, and practicality in service.