I am working on a project, 6-story residential building in an ovesea U.S. military base. 

At first, for the baseline, I used system-1, because in ASHRAE, it is stated that residential spaces in buildings are used primarily for living and sleeping.  They include, but are not limited to, dwelling units, hotel/motel guest rooms, dormitories, nursing homes, patient rooms in hospitals, lodging houses, fraternity/sorority houses, hostels, prisons and fire stations.  

However, this mech. engineer, who is a Mech. P.E., argued that the project shouldn't be considered as a residential, because in u.s. military projects, and U.S. building codes, consider residential building of 5 or more stories as a commercial building. 

I am not an energy-modeller, nor I have a MEP experience.  I am just managing the project, and responsible for preparing an energy plan.  Because of my inexperience with MEP codes, I couldn't argue with him, and my co-workers changed the baseline hvac system to system 7.   

If anyone can answer this question, I would greatly appreciate

and if you have any documentation that supports your answer, that would be great!!!

thank you