How do you specify frames of reference?

Answer 1

Frames of reference as defined for an observer :

REST :

If a body does not change its position with respect to the observer, then it is said to be "at rest" with respect to that observer.]

MOTION :

If a body continuously changes its position with repect to an observer, then it is said to be "in motion" with respect to that observer.

Answer 2

Normally poorly, due to incomplete understanding.

The key word is 'Relatively". Which means motion must always be considered relative to something else. i.e. The speed of a bus is not normally measured with respect to Andromeda, Mars, a nearby train, or the sea the ferry it may be driving on is sailing through, or the land under the sea. It is measured with respect to (wrt) the road, or ferry deck. Anything CAN be measured wrt anything else, but both must be specified.

If you don't mind failing your exam you could also tell the whole story; An 'inertial frame (considered a set of 3 'co-ordinates') is actually a real thing, i.e. a bus, plane, person on a bike, planet with atmosphere or galaxy. Light goes at a fixed constant speed ('c') within all inertial frames. Refraction at the boundary (fine structure/plasma) makes it so. Frequency (or wavelength subject to observer frame) Doppler shifts, conserving 'E' (energy). The 'rest frame' of the cosmic microwave background (CMB) proves it. Educational science has not put that together yet.