Linked from
The 10 pages that link to Magnetic quantum number, each with the reason it gives.
Principal quantum numberRelated: It adds an orbital label beyond the energy level given by n and l.
p orbitalRelated: Its three allowed values distinguish the three p-orbital orientations.
4f orbitalRelated: Its seven allowed values distinguish the seven 4f orbitals.
4f electronRelated: Seven possible values distinguish the seven orbitals in a 4f subshell.
Wigner–Eckart theoremRelated: The theorem isolates this label’s dependence in known coefficients.