What size drywall is required for commercial establishments. 1/2" or 5/8"?
It has to do with fire rating requirements. 

For NON-FIRE RATED requirements: Walls: 1/2” for 16” and 24” oc stud (wood or metal) spacing. Ceiling: 5/8” for 16” and 24” oc spacing. 

For 1- hour FIRE RATED requirements: Walls: 5/8” for 16” oc stud spacing. Ceilings: 5/8” for 16” oc spacing. If you have wider spacings, there are other options. Fastener sizes and spacing are based on the type of occupancy (use).

Doors and door frames need to be 1-hour fire rated in those walls too. Also, ducts passing through a fire rated wall must have a fire rated damper installed in the wall too. 

Fire rated construction can be required: 1) throughout the building, or 2) just on the corridor walls and ceiling, or 3) on party walls. You’ll need to ask the local Building Department when you get your Building Permit. You are getting a Building Permit...right?