Quantization in a narrow sense refers to discreteness of energy levels. In the context of atomic levels such discreteness is usually observed via optical experiments -e.g., via absorption, which has resonance at the frequency corresponding to the level spacing: $\hbar\omega = E_2 - E_1$. Studying optical absorption spectrum necessarily implies the use electromagnetic radiation, i.e. of time-dependent electromagnetic field.
Orbital momentum designates the states of a charge particle (electron) in the electrostatic field of the nucleus. As such these states are coupled to the electromagnetic field via charge, i.e. they are coupled to the electric field. In order to observe absorption on needs both constant and time-dependent fields: the former one to cause level splitting (via the Stark effect) and the latter to case absorption.
Spin is a kind of magnetic moment, i.e. it is coupled to the magnetic field, and consequently a constant magnetic field is needed to split spin levels, and a time-dependent field is necessary to cause absorption.
Finally, an experimental setup my use additional time-dependent fields to facilitate more precise measurements -e.g., by causing the Rabi oscillations.