Most of the given answers here are "not even wrong" because they address an irrelevant point. Obviously we're considering an object that is not moving in an external potential, so the question is how to interpret the difference of the total and the kinetic energy. The quantity mc2mc2 is the sum of the potential and kinetic energy of the system comprising of only the object in its rest frame. The total energy in a frame where it has a velocity of vv is γmc2γmc2, the kinetic energy of (γ−1)mc2(γ−1)mc2 refers to only the kinetic energy if its center of mass motion.

Note that splitting up the internal energy of mc2mc2 in kinetic part and a potential part is arbitrary. Take e.g. the van der Waals interaction between atoms. the total energy of two atoms a distance RR apart depends on RR, minus the derivative w.r.t. RR is the van der Waals force. While we can call this energy the potential energy of the two atoms, one can split it up in a kinetic part and a potential part when considering the electrons in both atoms.