Yeah, we discuss mass meaningful in rest frame. There are some differences in individual and systematic cases. Mass is equivalent to rest energy, and include all the necessary energy in the rest frame. So I think mass or rest energy is a perspective or concept to express the rest energy state. At the very beginning of universe, energy of other particles always exists when you stand on some particle reference system (if you stand on some photon reference system, own energy is 0), and nothing would exist independently. So from the system view, in the rest frame, rest energy always exists, which is just another landscape of energy by different reference system.Hi.
My intuitive is energy and mass are two different names for the same phenomenon.
Although single photon that has no rest frame has no mass but energy, photon gas that has its rest frame in the sense of zero total momentum has both mass and energy.