Complex sentence: Russian scholars attribute the high civilian death toll to the Nazi Generalplan Ost which treated the Soviet people as "subhumans", they use the terms "genocide" and "premeditated extermination" when referring to civilian losses in the occupied USSR.
Split into simpler sentences: Russian scholars attribute the high civilian death toll to the Nazi Generalplan Ost which treated the Soviet people as "subhuman". Contemporary Russian sources use the terms "genocide" and "premeditated extermination" when referring to civilian losses in the occupied USSR.