I've always had this as a note on my desktop.
Conc Beam Expl
Inft 100% 50% 25%
Vech 25% 100% 50%
Tank 50% 25% 100%
Vechs have 25% reduced health against inft. Tanks have 25% reduced health against vech.
The first bit states how much damage a weapon type does to a certain unit. For example if a concussive unit does 12 damage it will do:
12 damage to inft
3 damage to vech
6 damage to tank
The damage is never increased, always decreased.
Similarly the health is never increased, always decreased.
So when an inft unit goes against a vech unit, the vechs health will be decreased by 25% of its current health against that inft unit. Same with a vech unit up against a tank unit. A way to test this in the sim is this:
Expl vech armour (1) VS beam inft arm (6)
The dmg vech has 16 health, armour inft has a total of 12 damage.
If you increase the value of their damage by 25% (12*1.25) you get 15 damage, which WON'T kill the vech.
However if you decrease the value of the vechs health by 25% (16*0.75) you get 12 HP, which means the inft WILL kill the vech.
Running the sim you will see the vech will die in one round, which means the vechs health is reduced by 25%!!