How do attachments that add damage effects to projectiles work?

Flame = a very short duration DoT with an initial damage that does not stack, better used on fast firing weapons like pulse rifles or the plasma cutter.

Acid = a very long duration DoT that stacks, good on VERY fast firing weapons like flamers. A Few enemies are imune to this mod.

Shock = almost no damage but stuns the enemies for a few moments, good on high powered weapons that lack the stun effect like the sniper rifle (military engine+precision tip).

Damage support = increases the damage for about 5% for both players and stacks (10% for both), (this is an educated guess as no one knows the exact number, not worth using the flame gaze is more effective on any situation, the weapons that would benefit from this increase already kill everything in 1 shot like the contact beam, using an ammo box is way more effective for example).

Extra things you should know:

DoT stops knock back effects when it ticks, like the force gun, so avoid using these mods on Crowd control weapons, the stasis moule is designed for them.

Avoid using the stasis module on explosive weapons if you are not using safe guard, the stasis effect will also apply on you from your own shots.

Hope this helps.