_{Hello fellow players. I have a couple of question regarding the vehicle crew of the IG}

_{So our IG vehicles have some crew spots. Gunner, Loader and Commander ( hope i didn't miss something )}

_{What does each one actually do? Gunner is obvious but the Commander and the Loader? How about a vehicle assigned to the players... Are these vehicles come as crew spots filled or should be filled by the players? Can Comrades fill these spots too?}

I'll just preface this by saying that I'm drawing most of this from some WW2 games (Red Orchestra 2 and War Thunder, mainly) and some videos from ARMA.

The Gunner is, as you said, straight-forward. He has control of the main gun, any coaxial guns mounted to it, as well as the turret rotation. The Loader, as the name implies, reloads the gun; this means that the gunner doesn't have to look away from the target between shots. Since a Leman Russ seems to be able to fire multiple rounds before firing, I assume it has an autoloading mechanism - the loader's the one who grabs new shells off the ammo rack to refill the autoloader when it's empty. In vehicles with multiple ammunition types available, he has to pay attention any orders for specific ammo types; in WW2 that meant listening to the gunner or commander as to whether they wanted to fire a High Explosive round or an Armour Piercing one.

The Commander is the one who keeps the whole crew organised. His position has the best view of the battlefield and he's the one trusted to know where to put the tank and how to best employ it. In WW2 tanks, Commanders typically had the best view from their commander's cupola as it would have the most view slits or periscopes, affording them a far better view of the area. Modern tanks still do this as well; giving the commander the best vision equipment on the machine while the other positions are very limited in their visibility outside of the tank. He directs the Driver where to move the tank, the gunner where to point the gun, the loader on what ammo type to load and so-on. If he spots a target or a likely position for hostiles to approach from, he'll tell the Gunner to point the gun in that direction. He'll tell the driver how fast to go, what direction and whether to angle the tank against incoming fire (presenting a smaller profile to the enemy and forcing their shots to hit at a shallower angle, increasing the chance that the armour will deflect the shot). There was a good youtube video of ShackTac (an ARMA group) doing a mission with tanks that was from the view of one of the tank commanders. I'll edit it into this post if I can find it again.

EDIT: Here we go - this is from the perspective of a tank commander in a game where each side had a tank supporting their infantry platoon:

ShadowFighter88 already covered the first part, and I really have nothing to add there - but as for the second part of your question:

How about a vehicle assigned to the players... Are these vehicles come as crew spots filled or should be filled by the players? Can Comrades fill these spots too?

I think this really is up to you, or rather, your players! Talk it over with them, and see which option sounds the most fun to them:

RAW assumes you assign 1 tank for the entire squad, but don't let that stop you. The game is what you make of it, and unlike with a PC game, here you have full control and can change anything you want.

If you do want to play it by the book, and taking a Leman Russ MBT as example, you'd probably want all your players to pick the Operator class, and then have 1 assume the role of commander, 1 as driver, and distribute the rest on the 1-3 auxiliary weapons. Note that the hull weapon gunner is intended to double as loader for the main gun! This is probably why FFG wrote their Leman Russ with an autoloader; else the Loader would have a rather boring task most of the time.

If you have any unused slots left, fill them with Comrades. Leftover Comrades are discarded.

Alternatively, if you have enough players, you could try to fill two tanks with a mixture of Players and Comrades. Technically, you could even give every player their own Leman Russ -- there was a thread some time ago if you want to go down that route.

I gave my players a Leman Russ with a supporting Centaur APC. Comrades couldn't make attack rolls without the Gunner advance but otherwise could fill sponsons to give the Ranged Volley order or act as loaders (if the Heavy's comrade) and so on and so forth.

Note that the hull weapon gunner is intended to double as loader for the main gun! This is probably why FFG wrote their Leman Russ with an autoloader; else the Loader would have a rather boring task most of the time.

Which is why the Loader is the only non-playable crew position in Red Orchestra 2's tanks. Well... okay, technically you can play as the loader in that Russian light tank they added in an update but then you're also acting as the tank's gunner and commander because it's only big enough to fit two people.