What does this equation mean?

Holding $\mathbf b$ , $\mathbf v$ , M , and $\langle$ $H_{\rm NR}$$\rangle$ fixed and sending c$\to$$\infty$ , the second term vanishes and the first survives unchanged, converting a state-dependent relativistic phase into a fixed, universal constant multiplying every vector in the fixed- M sector — Bargmann’s central charge, reconstructed as the low-velocity limit of a quantity that was never central to begin with. This is exactly the contraction procedure Inönü and Wigner formalized for groups and their representations in general [ 3 ] , applied here to one specific commutator rather than asserted as a general slogan. The nonrelativistic mass superselection rule is, on this reading, the shadow a fully relativistic, non-superselected world casts onto its own c$\to$$\infty$ limit — a point developed independently in later work reconciling the rule with ordinary relativistic quantum mechanics [ 4 ] .