Single wall Metal chimney

I’m cuurently taking the Chimney course and It says single wall metal chimneys are not allowed for single family and 2 family dwellings? Then it lists air gap requirements for single wall chimney penetration.

Where exactly IS single wall chimney allowed if it can’t be used under any circumstance in single and 2 family dwelling? And why just 1-2 family dwelling requirements? What about 3 family dwellings??