It starts from Basic Ones like Setrange, Upgrade, functions, datetime transferfields etc. Then comes the intermediate level (You could actually see the change of level while you answer): report flow, data items, codeunits, automation. Then ends with skill + experience related like, integration, LIVE scenarios, application of NAV in different industries and few core technical ones.
Sigiloso
For a person having application knowledge of NAV, it's not difficult.