Safety Belts Regulation
 

I am looking into the government definition of when passengers should wear safety belts.

I know common sense says "Always".

The Canadian Air Regulation 605.25 (1) (c) says "at any time during flight that the pilot in command considers it necessary" but I can't find how that statement is defined.

When is necessary (according to the regulations)?

Is it only movement on the surface, turbulence and take off / landing?

Can anyone point the way?

FAR 91.107 and 135.128