have you ever noticed during the Captains blurb to the pax he is nearly always accompanied by Senior First Officer blogs, WTF is a senior F/O.
BA ( and many other airlines) have "Senior First Officer" as a specific "rank"/job title/job description. FWIW pilots start off (in BA at least) as a "two ringer", i.e. a "First officer" and then at some point get promoted up to a three ringer and become "Senior First Officers"...
Anyhow, no idea about "senior instructor".......I was an A2 at CFS but didn't hang around long enough to get senior.......