There is no rest mass for mass-less particles. When we refer to mass less particles we mean something without rest mass. For particles that have a rest mass, it is literally the mass of the particle that one would observe if you were to perform any mass experiment on it from the rest frame of reference (i.e. you are moving with the particle such that it looks like it's not moving with respect to you. So if you're floating in outer space, it is floating with you, or if you're going in a car moving at constant velocity, it is moving in the car with you. Theyre all the same).
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