Direct answer: Russian tar usually refers to tar associated with Russian leather and birch-bark processing, although the phrase can also appear as a loose trade name for eastern European tar products.
The most distinctive Russian association is birch. Russian leather was historically famous for a smoky, durable smell linked to birch tar or birch oil treatments. Because birch trees are common across northern Eurasia, birch-bark tar became culturally and commercially important.
Russian tar appears in discussions of leather, bookbinding, fragrance history, old trunks, military equipment, and traditional craft. It is less likely to mean ordinary pine tar unless the source says so.
The important distinction is between Russian as geography and Russian as material tradition. A phrase like Russian tar may point to birch tar, Russian leather dressing, or simply tar traded from the Russian Empire. Context decides.
In short: Often associated with birch tar and Russian leather. A trade and cultural phrase, not a single modern specification. Relevant to leather, fragrance, antiques, and bookbinding history. Context is needed to identify the exact tar.