In the particular sphere that I inhabit, we use three classifications:-
(1) Primary - if it fails, you're in serious trouble
(2) Secondary - if it fails, safety of the flight is endangered but should be recoverable.
(3) Tertiary - if it fails, it's annoying.
So taking a typical helicopter:-
Primary - rotor blades, Jesus bolt, hub mechanism, engine attachments, tailrotor linkages, gearbox, main airframe....
Secondary - main flight instruments, doors, windows, duplicated hydraulic components....
Tertiary - trim, placards, backup instruments, seat cushions, furry dice...
G
(You can have fun sometimes arguing over the boundaries when classifying things for safety analysis purposes - but in general, if in doubt class upwards).
(Good grief, this is my 4,000th post. I need to get out and do more flying!)