Special Russian Garrison Rule
Russia must maintain a Western Garrison of at least three Army (10) Units in Russia and the following Areas: Serbia, Rumania, Greece, Bulgaria, and Anatolia. If any of these Units are destroyed, the Russian player (if any) must rebuild them as soon as he can afford to do so. If Russia has fewer than four Army (10) Units, none can ever be moved to any Area outside those specified (although smaller Army Units can be moved).
I take this to mean (and have always played) that the Tsar is never going to leave the western frontier without a powerful Army. I suppose it could be read to only be applicable during M/SC, especially in light of the specific wording of the Indian Garrison rule, but I'd want to play with a Russian Garrison required to be ready to take advantage of/defend against Austrian or Turkish weakness/ambitions.