It is a mix of heavy sour oil of the Urals and the Volga region with light oil of Western Siberia.
[1] Other reference oils are Brent, West Texas Intermediate and Dubai.
Urals oil futures trade on Moscow Exchange.
[2] There was also an effort to trade it on NYMEX under the name of REBCO (Russian Export Blend Crude Oil).
[3] Urals grade oil was traded in Northwestern Europe on June 25, 2020, at a premium to Brent of $2.35/bbl – a record in the entire history of monitoring since September 1994.