The term bulkhead has many definitions, but in cabinetry it refers to two different aspects, both dealing with upper cabinets. Bulkhead size is mainly a matter of preference and design.
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One meaning of bulkhead is the space between the top of the upper cabinets and the ceiling. With standard wall height being about 96 inches and standard upper cabinet height being 30 inches from the bottom of the cabinet to the top, you will have about a 6-inch space above the cabinet when hung at standard height. Upper cabinets range in height from 24 inches to 42 inches, so you could choose to have no space at all or a larger one for taller walls.
The other meaning of bulkhead is the cutout in the ceiling for the tops of the upper cabinets to fit into. In this configuration, there is no space between the top of the upper cabinets and the ceiling. The standard bulkhead, or soffit depth, in this case is at least 13 inches. Minimum depth of upper cabinets is 12 inches, allowing for a ¾-inch trim piece between the cabinets and the soffit.
Cabinets can be custom designed to any dimension, with a main determining factor being ceiling height. Design style may provide for a large space on top for added décor, while no space means less dusting.