Question-and-Answer Resource for the Building Energy Modeling Community
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As a more general explanation, when you're designing HVAC systems that serves multiple zones, you want to make sure that all the zones being served have similar loads and schedules. Otherwise, you end up running the entire system just because of one zone, which wastes energy. Because you sometimes have zones that are outliers, instead of putting them on the multi-zone system, you give them their own system. Another reason to put a particular zone on a separate system is if that zone is physically far away, and it wouldn't make sense to run ductwork/piping that distance.
Here's what this logic looks like when applied to the Secondary School Reference Building:
You can see this logic described pretty concisely in ASHRAE 90.1-2010 Appendix G:
"...If the baseline type is (a multizone system type), use separate single-zone systems...for any spaces that have occupancy or process loads or schedules that differ significantly from the rest of the building. Peak thermal loads that differ by 10 Btu/hr*ft2 or more than 40 full load hours per week are considered to differ significantly..."
