According to the Pauli Exclusion Principle, no two electrons in an atom can have the same set of four quantum numbers. If two electrons occupy the same orbital, they already share the same Principal ($n$), Azimuthal ($l$), and Magnetic ($m_l$) quantum numbers. Therefore, they must be distinguished by the fourth quantum number, the Spin quantum number ($m_s$), which must have opposite values ($+\frac{1}{2}$ and $-\frac{1}{2}$) (anti-parallel spins). This concept is fundamental to determining electronic configurations and magnetic properties (paramagnetism/diamagnetism) of elements.