Many codes require an S-Label for certain doors in some occupancies. For instance, corridor doors often require smoke protection to prevent smoke from leaking from or into an occupied space.
An S-label isn't exactly a "smoke rating". Rather, the S-label indicates the door meets the leakage tests that are part of UL 1784: Standard for Air Leakage Tests of Door Assemblies and Other Opening Protectives. However, smoke cannot pass through if air cannot leak through the door assembly. An S-label is usually achieved by installing UL approved gasketing.