A light boson U from an extra U(1) interpolates between a generalized dark photon coupled to Q, B, and Li (or B-L), plus possibly dark matter, a dark Z coupled to the Z current, and one axially coupled to quarks and leptons. We identify the corresponding U(1)F symmetries, with F=γYY+γBB+γLiLi+γAFA+γF′F′+γdFd, Fd acting in a dark sector and F′ on possible semi-inert Brout-Englert-Higgs (BEH) doublets uncoupled to quarks and leptons. The U current is obtained from the U(1)F and Z currents, with a mixing determined by the spin-0 BEH fields. The charge QU of chiral quarks and leptons is a combination of Q, B, Li, and T3L with the axial FA. It involves in general isovector and isoscalar axial terms, in the presence of two BEH doublets. A longitudinal U with axial couplings has enhanced interactions and behaves much as an axionlike particle. Its axial couplings gA, usually restricted to ≲ 2×10-7  mU(MeV), lead to effective pseudoscalar ones gP=gA×2mq,l/mU=21/4GF1/2mq,l A±. A± is proportional to an invisibility parameter r=cosθA induced by a singlet v.e.v., possibly large and allowing the U to be very weakly interacting. This allows for a very small gauge coupling, expressed with two doublets and a singlet as g′′/4≃2×10-6  mU(MeV) r/sin2β. We discuss phenomenological implications for meson decays, neutrino interactions, atomic-physics parity violation, naturally suppressed π0→γU decays, etc. The U boson fits within the grand unification framework, in symbiosis with a SU(4)es electrostrong symmetry broken at the grand unification scale, with QU depending on Q, B-L, FA, and T3A through three parameters γY, γA, and η.