Licences. The following is basically how the categories are. P | Aeronautical / Aerospace Engineering.

Licences. The following is basically how the categories are. Please research 'more' into your interest.

Cat. A - Airframes.
(Research on sub licences B1.1 or B1.2)


Cat. C - Powerplant/Engines (Propellors)


Cat. X - Avionics.
(Generator, Autopilot, Instruments, Compass - Couple and Non Couple to flight direction)

Cat. R - Radar.
(Inflight and Ground.)

You can confirm these by checking out:
1. Your national aviation authority website
2. https://www.icao.int/Pages/default.aspx
3. https://www.faa.gov/
4. EASA parts
5. SR technics