Major Bill, from what I read in the latter years, a Corps commander such as Q. A. Gilmore would request an area of operations to the Army Commander, or Halleck, the chain of command to the secretary and to the President. In this case Grant's decision on the Tenth Corps as Gilmore's preference to relocate his command to a more active district was granted. Gilmore had requested, by outlining his strength in numbers available, and what could be left behind to safeguard Hilton Head operations. His case was possibly special, being that General Seymour screwed up his entire campaign, leaving him with a miscarriage of operations. He wanted another chance to prove his Corps and leadership. The channels of command all approved because he did his paperwork and submitted detailed possibilities, good morale, and separate requisition for sub-commanders such as Hatch, on a time schedule that did not interfere with current ops, and was wise with the Navy Transport system. So division commanders could come by request, and they in turn (div. com.) could request their Brigades. Chain of Command approval upon active, on-going campaigns. Replacements were necessary in this case.
Lubliner.