You're right about that, Colonel. The Navy has a sort of ambiguous class of officers called "Limited Duty Officers" (LDO). These men are commissioned, and enjoy all the rights and privileges thereof, but their duty and authority is limited to a specialty area, e.g., nuclear propulsion. I don't know how these officers are selected these days, but back when I was around, the usual source for LDO's was a program called NESEP (Navy Enlisted Scientific Education Program), wherein the Navy sent highly qualified enlisted personnel to college, where they could earn a Bachelor of Science degree, and a LDO commission, in return for six years (I think) active duty.

To bring this back on topic, staff officers in the Confederate army held commissions but had no command authority over line troops (there, that should satisfy Jim!).