How Did The Sanitary Commission Do Its Work?

[Good Soldier Shweik]

How close did they get to the front lines? If the army settled in place, how soon was the sanitary commission there? Would there be commissioners be at Chattanooga at any time that Rosecrans was there? When the cracker line opened?
Did they show up after a major battle? How soon? I am reading that a famous actor was trying to get his son's body after Chickamaga, and there was a lot of beaurcracy involved in getting it, and he did a USO tour of the camps.

Thank you