Damage: Damage that you dealed. deactive it in mod options to avoid conflict if you are using vanillahudplus.

Heal: Your self healing.

Mitigation: Damage that your armor true blocked.
If your armor is 50 and take 20, 225 damage in streak, then your mitigation is 225 - (50 - 20) = 195, Because that 195 damage not affected you.