In a textbook I am using, the homonuclear Molecular Orbital Theory Diagrams label anti-bonding orbitals with a (*), and label both corresponding bonding and anti-bonding sigma-orbitals with the same number (e.g. 1σg then 1σu*).
The heteronuclear MO Theory Diagrams, however, don't always label the anti-bonding orbitals with a (*) and label the bonding and anti-bonding sigma-orbitals with sequential numbers. (e.g. 3σ then 4σ*; or 3σ, 4σ, 5σ then 6σ).
I've attached pictures to make it clearer.
I understand why the parity labels aren't used, but I don't understand why the numbering system changes or why the * isn't always included.
If anyone could explain this I would be really appreciative, thank you!