When we have to choose, given $n$, $\ell$, where to put an electron, when there are several $m$ orbitals, with the same energy, is there a concrete quantum mechanic rule or some superselection rule to choose? Example: 1s²2s²2p³, is the last electron $m=+1$, or $m=-1$? If it were 1s²2s²2p⁴, same question...How to choose? In some places I find a rule of "maximum $J=L+S$", and sometimes I have seen however the place the electrons from minimum $m$ to maximum $m$. I suspect this is conventional in the last case, unless there is some interaction that breaks down the degeneration in energy. Are the schemes $LS$ or $jj$ coupling related to this?