In Red Army in WW2 it was divisions that were rotated out. Preferred method was that worn out unit was withdrawn from the frontline and replaced by a fresh equivalent as this allowed higher formations to maintain same force on the actual front. So infantry division was pulled back and fresh infantry division took over their section of the front. Red Army insisted on units being brought up to strength away from the front rather than individual replacement on the front itself. This allowed new recruits to be better integrated into existing formation which reduced their casualties in later combat. It also allowed veterans to get some rest as prolonged combat reduced effectiveness of the unit. However unlike German military Red Army kept such units relatively close to the front so they could be used as a reserve in the case of an emergency.

After 1942 additional step was taken in that units were not disbanded as this preserved the existing cadre of officers, NCO's and specialists. Of course this was not the case in 1941-42 as encircled units were frequently destroyed and very few soldiers escaped, not enough to rebuild the division around them.

Stain's Keys to Victory: The Rebirth of the Red Army in WW2, Walter S. Dunn jr, 2006