Explainer-Why does Russia want to capture the strategic Ukrainian city of Pokrovsk?

MOSCOW/KYIV (Reuters) -Russian forces are starting to encircle the strategically important eastern Ukrainian city of Pokrovsk after capturing a string of villages to its south, and Ukraine has halted production at its only coking coal mine nearby. Following are key facts about Pokrovsk, which Russians call by its Soviet-era name of Krasnoarmeysk, and the battle for control. WHAT IS POKROVSK?