Good for you getting custom cabinetry. Do you have any idea how many people post here to ask what they can do with the particleboard that fell apart in their cheap production cabinets.
3/4" hardwood plywood is used on the sides and bottom of most cabinets. This allows for shelves or frames to be joined with rabbets or dados. The extra thickness is important for cutting and joining It is also much more stable. On most cabinets, the sides are the main structural supports. On the other hand, some cabinets have no back on them, which is fine. If you have a carpenter making your cabinets, I'm sure he will select appropriate lumber.