Lepton number

It is in the course, and it is easy to miss because it appears inside a list. Essential knowledge 15.8.A.2.i: in all nuclear decays, nucleon number (the number of neutrons and protons), lepton number (the number of electrons and neutrinos), and charge are conserved. Three conserved quantities, and that parenthesis is the CED's only gloss on this one.

Take the gloss literally and beta-minus decay fails to balance. A neutron changes to a proton, emitting an electron and an antineutrino. Counting "electrons and neutrinos" gives zero on the left and two on the right. The books balance only with the standard convention, which the CED never spells out: each lepton counts $+1$, each antilepton counts $-1$. Then the left side is 0 and the right side is $(+1) + (-1) = 0$.

Run beta-plus decay the same way. A positron is an antilepton at $-1$ and the neutrino beside it is a lepton at $+1$, so again the sum is 0 and again the parent nucleus started at 0.

That is the practical use of the rule: it is what forces the neutrino or antineutrino into the equation. Charge and nucleon number are already satisfied by the daughter nucleus and the beta particle alone, so nothing but lepton number tells you a fourth particle is there.

Scope stays small. The Topic 15.8 boundary statement excludes types of neutrinos, so no muon or tau family and no separate lepton numbers for each. Baryon number, strangeness and the rest of the conserved-quantity zoo are not AP Physics 2 either.