Type IIB vs. Type IIIB

I am working on a 5-story, 65 foot tall apartment building in California- 5 stories (plus penthouse mezzanine residential )over a basement Garage. It is a non-combustible concrete building with metal stud exterior and interior walls. Its per-floor and total areas are well within the limits of Table 503 for either of these Types, as are stories and height, once one factors in the height increase per Section 504.2.

This a new Type for me, and I hear a lot about Type III or Type III modified construction being employed for these type of buildings. As I look at the code, I am wondering why this wouldn't just be a Type IIB building. Are there advantages to pursuing Type IIIB vs. Type IIB, things I am missing?

Type II allows for non-rated, non-combustible exterior if adequately away from property line, Type III requires 2 hours. Just seems like IIB is the best calssification.

Any thoughts?
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