For the past couple of days, I've been reading "My Dear Friend: The Civil War Letters of Alva Benjamin Spencer, 3rd Georgia Regiment Company C," edited by Clyde G. Wiggins, III. Spencer was in the band and, while he witnessed the horrors of the war, was not often on the front lines. But he does give us some clues about the inner workings of the army. At the time of the Overland Campaign, they were members of Anderson's division, Hill Corps.
On April 22, 1864, Spencer penned a letter to his sweetheart back in Georgia. He was writing from Camp "Jennie Hart" on Madison Run. "A few days since we received orders from Genl. Lee," Spencer wrote, "that should any of the brave soldiers of this army be so unfortunate as to fall into the hands of the enemy, they should not tell to what brigade, division or corps they belonged; but simply give their names, company and regiments; also prisoners should not talk with each other in reference to anything connected with the army.'" Like all good soldiers, Spencer and his compatriots tried to determine what the order meant and came to the conclusion that "Longstreet's corps is undoubtedly at Charlottesville." (110)
Those of us who grew up right, watching old war movies on Saturday afternoons, recall POWs only giving their name, rank, and serial number. Of course, Confederate (and Union) soldiers did not have serial numbers. I've tried to find Lee's order, but I've not had much luck yet.
Longstreet and most of his corps were shipped to Georgia in September 1863 (passing through North Carolina). He was instrumental in the Confederate victory at Chickamauga, and the Confederate defeat at Chattanooga. After a failed attempt to re-capture Knoxville, he spent the winter of 1863-1864 in east Tennessee. With the opening of the spring campaign in Virginia, Lee wanted Longstreet back with the Army of Northern Virginia.
Lee actually planned to use Longstreet to attack the Federals positioned across the Rapidan River (see Lee to Davis, April 25, 1864, Official Records, Vol. XXXIII, 1282-83). Based upon Spencer's letter home, Lee was trying to keep Longstreet's movement a secret.
Name, rank, and regiment. I don't recall reading this in any other source.