You see, players have a bunch of skills and tool to cause status effect that do dot damage (damage over time) mainly both Burning status effect (that causes fire damage, but fire damage and burning status effect are different things) and Bleeding status effect that as far as i know always deal the same dot damage no matter the source (frag granade bleed is the same as the crossbow and seeker mine bleed).

But many of burning status effect many of those tools do drasticly different amounts of damage initially and also drasticly different amount of dot fire damage, and some of them like firestarter actually first cause explosion damage AND burning status effect, while airburst explosion is actually pure fire damage then it causes burning status effect, both of which you can simply see the yellow number on the explosion of firestarter and the orange number on the airburst explosion.

You see, fire damage actually works against machines and other stuff, what doesn't work is burning status effect, so you actually have very interesting interactions due that, flamethrower does pure fire damage and does cause burning status effect, you should be be able to hit machines with the flamethrower raw damage but won't add the dot to them, meanwhile enemies molotovs do cause both fire and burning status effect, same for outcast elite rushers fire pool which is quite interesting since the burn dot that you get from it it's quite low actually, the fire damage from standing in that fire, it's close to few millions so if you kill an elite rusher in a spawn point, it will kill all other elites as they spawn, it will even kill heavies extremely fast. Now our incendiaries granades do actually 0 fire damage but create a pool of fire that will cause 100% burn stauts effect to anything that isn't immune to it, so enemy granades do 2 kind of damage to us (fire and burning dot), ours only cause the burn dot.

Now this part is something that i am not sure really sure so i would like someone else to confirm it for me, fire damage (aka flamethrower and airburst) do armor damage as any other regular bullet which i admit haven't tested much so could have been patched this last week or something, explosion damage as most would know bypass armors from certain enemies such as the robo dogs, that means that firestarter is drasticly better than airburst and flamethrower to deal with enemies, since it works great against machines due explosion damage as well as regular enemies due explosion + burn damage, so if you are dealing with blackstusk and doing fire build (and well acutally against any faction really) firestarter chem launcher is your friend.

Now you see, bleed is actually completely consistent, it will do a low dot damage no matter the source, the dot damage will always be the same, while it's affected weirdly like using AR to cause bleed on a target will increase the bleed damage when it reaches their health do health damage bonus from the AR, but there is no source of bleed status effect that will cause a bleed that is drasticly stronger than the other (such is the case of firestarter vs airburst seeker).

Quick mention to shock status effect that actually causes damage to machinery if you can somehow cause that (special ammo, shock traps and other enviroment hazards), but it's damage is so low that it shouldn't be counted, same for jammer doing to machinery like black tusks drones, it jsut does a big wallop damage against not the instantly destruction is not guaranteed if you play legendary.

tl;dr: this is just a wall of text about weird interactions of different status effect with damage and other stuff. Burn>>>Bleed