Short answers, pulled from the story.
Southern Russia is a colloquial name for the southernmost portion of European Russia, generally referring to the area covered by the Southern Federal District and the North Caucasian Federal District. The term carries no official status under Russia's administrative classification system.
Southern Russia spans two of Russia's nine federal districts: the Southern Federal District and the North Caucasian Federal District. Together they encompass 19 federal constituent entities.
Southern Russia does not appear in the Russian Classification on Objects of Administrative Division because it is an informal geographic term, not a formally designated administrative unit. Russia's official federal structure divides the country into federal districts and constituent entities, none of which bears the name Southern Russia.
South Russia was a military quasi-state that existed during 1919-1920, at the height of the Russian Civil War of 1917-1922. It claimed territory across the southern Russian Empire, including parts of present-day Ukraine, and was defended by the Volunteer Army and the Armed Forces of South Russia. Soviet historians also called it the White South, referencing the White Army that fought the Bolsheviks there.
The Mongol invasion of the Caucasus during the Late Middle Ages brought Turkic vassals who established a Tatar state between the Volga and Don rivers. Historical sources record that the region's population converted to Islam following contact with the Mongols, and that the area underwent a broad process of Turkification.
Six of Southern Russia's 19 constituent entities are disputed with Ukraine. They are the Republic of Crimea, the federal city of Sevastopol, the Donetsk People's Republic, the Luhansk People's Republic, Kherson Oblast, and Zaporozhye Oblast, the last four of which Russia occupied and claimed from 2022.