Soviet Union key features are "modernization" and "leader abilities". Modernization is needed in order to request more advanced vehicles. There are 2 levels of modernization for each unit category. For example tank modernization 1 unlocks the T-34 mod. 1943 and tank modernization 2 the T-34/85.
Some units, such as the Militia Commisar, are considered "leader" units and will unlock specific abilities in certain situation. For example Militia Commisars within 15m from a friendly unit can execute it to motivate the other men. The related "leader ability" is unlocked in the proper menu whenever a commissar is in range of this ability.
The Support operations for the Soviet Union will be the following:
T-34/57: You will receive a modified T-34 armed with a 57mm ZiS-2 cannon.
IS-1: You will call in an IS-1 heavy tank armed with a 85mm cannon
T-34 Command tank: This T-34 unlocks the leader ability "Armor replacements" in the tank leader abilities.
KV-2: You will call in a KV-2 heavy tank armed with a 152mm howitzer. This unit unlocks the leader ability "KV-2 barrage" in the artillery leader abilities.
Not a step back!: Your soldiers won't be controllable but they will gain a huge health bonus
NKVD squad: This squad unlocks the leader ability "Zealous devotion" in the infantry leader ability
Elite Sniper: You will receive a squad made by 2 snipers and 1 elite sniper.
Katyusha: You will call in a couple of katyusha rocket launchers.
Partisans: You will receive a squad of Partisans armed with captured German weapons and incendiary bombs.
Incendiary Bombs: Every incendiary bomb laid down by a Partisan will detonate instantly.
Cut supplies: Each enemy vehicle will have a small chance of being damaged every 30 seconds.
KV-8: A modified KV-1 armed with a coaxial flamethrower and a 45mm cannon will support your troops.
Soviet War Machine
Raw materials: You will receive 5 random tech points.
Double Production: You will receive 2 tanks when you request a tank reinforcement
Land Lease: Call in a couple of M4A2 Shermans escorted by soviet tank hunters armed with M1 Bazookas.
Red Wave: Every second you will receive a random infantry or tank reinforcement independently from your current modernization level (this means you can receive an IS-2 even if you are at tank modernization level 1).