This study begins with the recognition that the Standard Model particle-antiparticle classification lacks a universal physical principle that determines the particle orientation of each charge-conjugate pair while grouping the proton, neutron, and electron into the same particle sector. Motivated by the weak-isospin doublet structure of weak fermion transitions, we introduce a new weak-isospin-based quantum number, N=2T_3L, where T_3L is the third component of left-handed weak isospin. States with N > 0 are assigned to the particle sector, states with N < 0 to the antiparticle sector, and states with N=0 to an N-neutral sector. In this classification, the proton and neutrino are particles, whereas the electron and neutron are antiparticles. The representative weak, hadronic, stellar, and early-Universe processes examined here consistently conserve the total additive N while redistributing N among different species. For fermion species defined by their left-handed weak-doublet orientation, the electroweak charge assignments give N=2Q - (B - L), where Q, B, and L denote electric charge, baryon number, and lepton number. Hence, for charge-conserving processes, exact additive N conservation requires Δ(B - L)=0. Accordingly, the same Universe that appears strongly particle-antiparticle asymmetric under the conventional classification is reinterpreted in the weak-isospin-based classification as a Universe in which positive-N and negative-N components cancel globally, yielding the global N-symmetry condition N_tot=0. This work further proposes a possible particle physics mechanism for the species-level particle-antiparticle and baryon asymmetry problems. A proposed N-conserving 1-->3 decay process such as "n̄ --> p + e- + ν_e" can generate, in a single decay event, the asymmetry direction required to account for the present abundance pattern, (δ∆p, δ∆n, δ∆e, δ∆ν) = (+1, +1, +1, +1). Related 2 --> 2 scattering processes such as "n̄ + e^+ --> p +ν_e" and "n̄ + v̄u2091 --> p + e-" produce the same asymmetry direction. As a possible microscopic origin of the required CP-conjugate rate asymmetry, we examine fermion chiral structure, left-right asymmetric interactions, and CP-violating interference between distinct amplitudes. Accordingly, this work recovers global particle-antiparticle symmetry through the N=2T_3L classification and proposes a possible solution to the species-level particle-antiparticle asymmetry through new N-conserving particle-conversion processes. By predicting decay and scattering channels forbidden in the Standard Model, the proposed mechanism can be tested directly through searches for N-conserving 1-->3 decay, 2-->2 scattering, and their CP-conjugate rate asymmetries.