Not knowing your ancestor's name (what was his name?) and knowing what is in his compiled service record I cannot look at what records have survived but some facts and assumptions can be made.

1. He was left at Tazewell, Tennessee due to disease or injury. It is unlikely due to a wound because the 56th Georgia was not involved in direct combat and did not see action at Perryville. Depending on the disease or injury he could have been incapacitated for weeks to months.
2. IF there is no surviving record between the time of October 1862 and the men reporting back to parole camps in September 1863, then any number of scenerios could have taken place. The seige of Vicksburg was from approximately May 20-July 8, 1863. That means somehow or another he did not, could not, would not go to Vicksburg prior to the middle of May 1863 to report to his regiment. If he was fit for duty he would have been directed to go to join his regiment which was being transported by train to Vicksburg during the battle of Murfreesboro. If he had been fit for duty, he would have already been directed to report to a parole camp to await the regiment's official exchange and then the regiment would have rendezvoused with the rest of Cumming's Brigade at Stone Mountain and then Decatur for reequipping.
3. Typically, soldiers that were medically retained were put on medical furlough and sent home or retained at the hospital. If the men were fit for duty but could not return to their regiment because of being cut off from them as in this example, then they were sent to a Camp of Direction to be held until they could reunite with their regiment. A Camp of Direction is/was used to gather pieces of units or individuals and gain some type of positive control over them while they are awaiting transportation to their parent organization.
4. It would have been practically unheard of for an individual soldier who was geographically from Campbell or Coweta County, Georgia (where he would have been on sick furlough if not in a military hosptial) to find himself individually put into the ranks or assigned to an organization other than his own. A Camp of Direction would have ensured that he was united with his parent organization once the regiment was located and if they were cut off as in this case the men would have been retained by military authorities.
5. This does not take into consideration that he may have been absent without leave and could have been out of military control and then returned to the ranks.
6. The combat on Missionary Ridge was dynamic, however, the Confederates were pretty dominate in controlling the ground on the north end of Missionary Ridge [were the 56th Georgia and Cumming's Brigade was at] and this action was a Confederate victory were Cumming's Brigade played a significant role. Was he wounded at Missionary Ridge?

You may want to see if he filed a Confederate Veteran Indigent Pension in the postwar years or other statement he may have made accounting for the time. Did he make a statement on someone else's pension request? Occasionally they will say something to the effect, "I do not know because I was...."

I hope this helps. Please let me know if you have any questions.

Respectfully,

Gerald D. Hodge, Jr.
M.A. Military History - Civil War Concentration
War Between the States Historian
Research - Preservation
Historian: 39th Georgia Volunteer Infantry Regiment