is the total angular momentum, 
 is the azimuthal quantum number, 
 is the spin quantum number, and
 is the secondary total angular momentum quantum number.
Which transitions are allowed is based on the hydrogen-like atom. The symbol  is used to indicate a forbidden transition.
In hyperfine structure, the total angular momentum of the atom is  where  is the nuclear spin angular momentum and  is the total angular momentum of the electron(s). Since  has a similar mathematical form as  it obeys a selection rule table similar to the table above.