We all know that no matter how it is all about the merc and not about the class, each merc still has a class associated with it.
Looking at how Splash Damage classified the mercs, what features or roles defines a specific class in Dirty Bomb?
The way I see it, the classes in Dirty Bomb closely match the description for the classes in Wolf ET.
Now another question, with the way you define the features and roles of each class, is Phantom a recon merc?
The description for Covert Ops mentions infiltrating and Phantom can infiltrate, meaning he is a recon merc.
Assault mercs all carry heavy weapons.
Medic can heal and revive
Engineers are objective specialists with a trap or ambush like ability that is effective more on defense than on offense.
Fire support give ammo and have an area denial or an area of effect ability with long cooldown.
Recon specialize in stealth, or combat with the enemy without the enemy noticing you, and are better at seeing what the enemy is doing without the enemy noticing.
The descriptions of the classes above fit all 20 mercs including Thunder, who carries a LMG that reduces speed by 82% of base speed like every other heavy weapon, Turtle who is an objective specialist, I assume, and has a shield which is better on defense. Aimee has a sniper rifle, which can shoot enemies from afar and can look at enemies with third eye cam.
Maybe the classification might change but so far, this is how it is with the first 20 mercs.