I did a Goggle search and seem to find an Exchange Battalion in Louisiana and Alabama.

I suspect it was an organization collecting soldiers exchanged and awaiting further assignments.

I find this statement, but have not been able to follow up on it:

Exchanged Battalion, C.S.A. (Trans-Mississippi Battalion; Western Battalion)

Here is one I did find:

Jarnagin, Hamilton, Private/2nd Sgt

After the War (about 1870) he moved to Texas. He wrote "I will never forget the hardships, and privations that we have taken together." He was married to Griselda Orlena Kirby [b. December 22, 1846 Jackson County, AL], d/o Richard Lawrence "Sauta Dick" Kirby and Elizabeth Jane Gross. Hamilton & Orlena were married on July 6, 1868 in Jackson County, AL. This was the same Hamilton Jarnigan serving in Exchange Battalion Co. D CSA. He was the son of William and Elizabeth Jarnigan of Jackson Co. AL. Hamilton died on Jan. 15, 1908 near Lewisville, Denton Co., TX. He had lived in TX for 43 years. His wife, Orlena, applied for and was granted a widow's pension on July 16, 1914 at the age of 67. File #28705. Hamilton's brother Franklin, age 69 and living in Pisgah, Jackson Co., AL, filed a deposition on July 30, 1914 to aid Orlena in receiving the widow's pension. Orlena died on June 21, 1935 near Lewisville, Denton Co., TX, at the home of their only child, William L. Jarnigan. William and Elizabeth Jarnigan are buried at Old Sardis Church in Jackson Co., AL.

Hamilton Jarnagin was enrolled in Co. B, DeKalb Rifles, 49th Alabama Infantry, he was captured at Port Hudson July 9, 1863 and paroled there July 12/13, 1863