Plans and flies the tests that expand what an aircraft is cleared to do.

Flight test engineers design the test points, brief the crew, sit in the aircraft or the telemetry room, and expand the envelope one careful step at a time. The discipline exists because the alternative is finding the edge by accident.
Aerospace or mechanical, then a flight test organisation.
A well-worn route, particularly for test pilot school.
Aerospace engineers · US Bureau of Labor Statistics, Occupational Outlook Handbook, May 2024 wages and 2024–34 projections.
Counted inside "aerospace engineers", along with every other engineering discipline that works on air and space vehicles. There is no separate published line for this specialism.
Flight test is deliberate exposure to the edges of what an aircraft has been proven to do, and the discipline exists because people died establishing it. The hours are long, the deployments are real, and the job is to be extremely calm while being extremely careful.