There's a difference between ballistics shields and normal shields, which I'm hoping is incredibly obvious.

C'mon, guess. I know you're reading this primer manual or whatever while shamming on duty or taking a shit, so you've got time, give it a real long think on what the difference is.

Yeah, correct, it's because one shoots back while the other is just handheld cover. Good job legionnaire, you're so smart, please never get promoted otherwise you'll end up like me typing this shit at 01:35 because your commanding officer is an unreasonable whore who demands you WRITE OUT AN EXPLANATION AS TO WHAT A FUCKING SHIELD IS.
_{Says the orangutan throwing a bitch fit over something that'll take 10 minutes at most. – Iza}

I’ll start with the actually interesting one.
Ballistics shields are what happens when you take a concept and say, "Fuck it, we have funding so why not".
The main idea behind ballistics shields is that they function as "defensive offense", an implement of protection that can be used to strike back. Personally, I think it's cool as hell. Also personally, I have no idea if I'd ever use these in an active war zone.
There's a few ways this weird ass piece of tech works.

First, a ballistic shield can reflect projectiles back towards an adversary. This can be achieved a number of ways, such as engineered materials or via the cheater's way (gray conduits).
Second, a ballistics shield can use accumulated force to strike back with a pulse cannon type implementation. This means the longer the bearer sustains fire, the stronger the concussive blast will be when attacking.
Lastly, the shield can just straight up use aeroprene threads as its method of striking back.

Classification for ballistics shields is straightforward: the more integrated tech the shield has, the higher the class due to the expanded functionality. Now if I ever even issue these from the armory depends on the situation, but more often than not they're never needed. Pretty damn handy for missions that require all defensive loadouts, such as (God forbid) a contract job specifying we can't kill anyone.
Yet for nearly all other situations, I would remind the legionnaire they have a pistol. Just shoot the bastard.

Now as to what a normal shield is; it’s a big plank of some material that you hold up to stop someone else from ruining your day. It can take the form of dense fabric, wood, metal, polymers, ceramics, whatever mix of stuff is necessary to keep bad shit from perforating your body.

Sword or knife attack? Stop it with the shield. Gunshots? Take cover behind the shield. Explosion? Get behind the shield. It's a wall with handles that can be placed between you and something else.

Our usage of them hops all over the place.
Guards in high security places have them as an optional accessory, often paired with a handgun or sword/nonlethal weapon in places with sensitive equipment/chemicals. Assault Vanguards will take them into a fight when they're rushing into an enclosed area like a bunker. I've seen some Heavy Vanguards use shields alongside their main armament, like a Tectonic Rattler, but I'm not a fan of the accuracy they showed.

To a certain (small) degree, you can use a shield as a weapon since it has a handle. Closed in with the enemy? Bash them with the shield. Enemy knocked down? Use the edge of the shield to smash their neck in.

While they can be used as weapons, if you go into a fight with only a shield and come back alive somehow I'm going to throw you out of a window.

For shields we’ve got the following classes, despite the fact we hardly ever issue these out:

-FrW Nahli Lok-Riveria