Battlefield 2142 uses a modified version of the Battlefield 2 class system. Both teams (PAC and EU) have four classes, which are as follows. All classes contain a knife/Pistol and can use FRG-1 grenades and whatever squad leader unlock chosen. Each class has a specific purpose.
This is the sniper/recon class. It contains the sniper rifles (Morrieti, Park, Zeller), the Lambert Carbine, TI-33 Active Camoflage, RDX explosives, APMs, NetBat targetting link, DysTek enhanced scope, Gruber 5 Scope stablizer, and (Northern Strike only) Active Camoflage upgrade, and Decoy beacon.
Use this class to eliminate enemy soldiers from afar and sabotage the commander assets. The only CQC weapon these soldiers have is the Lambert. Be careful until you have it.
This is the Assaulting/Medic class. It contains the Assault rifles (Scar, Krylov, Baur, Voss), the Herzog Rifle Shotgun. the PK-47 rifle rocket, the AED defibulator, MHUB, NetBat class indentification, smoke grenades, and (Northern Strike only) Motion Grenade and attachment magazine upgrade.
These guys take the brunt of the attack. They also are the brunt of the attack. These guys are focused on taking enemy territory, clearing out the rooms, and making sure everyone is up and moving forward. Remember that the Herzog and the PK share ammo, so two shots fired from the PK=two shots fired from the Herzog. Keep an eye on your ammo reserves.
This is the Anti-Tank/Engineer class. It contains the Rocket launchers (Mitchell, Sudnik, Pilum, SAAW), motion mines, EMP mines, repair tool, SMGs (in place of pistols), NetBat vehicle indentification upgrade, AE diffuser, vehicle radar, repair tool upgrade, and (Northern Strike only) motion mine bait and SMG clip expansion
As the only class capable of taking down vehicles, you are a prime target for infantry. Keep your SMG out until an enemy vehicle is spotted. Also, pick your targets carefully, FAVs are not as good of a target as a Battlewalker. Deploy your mines where they are least likely to be spotted, like around corners, inside bushes, at the lip of a ramp etc. Those who use the Zeller or AE diffuser can take care of those mines with out setting them off.
This is the Defense/Support class. It contains the LMGs (Bianchi, Shuko, Ganz), the Clark shotgun, AHUB, sentry gun, Portable Shield, NetBat Active Camoflage Indentifier, DysTek pulse meter, EMP grenades, and (Northern Strike only) infantry radar, and RDX shotgun.
If you play as Support, you need to know that you are meant to support! Do not go solo into an enemy position, leave that to the Assault troops. You are supposed to provide security for an area or resupplies or any other form of support. Now, that doesn't mean you can't go Rambo, but that is not what you are meant to do. Battlefield is about teamwork, not badass-ery. For example, EMP grenades do not damage vehicles, they are meant to disable them, allowing your teammates to kill it. Also, the Pulse meter, that shows you where the enemy is within a certain radius of you FOV. Use it to help prevent your sniper from being killed, your engineer from being knifed etc...