Taken from TINY DICE DUNGEON [WIKI]:
For "regular" damage (Physical and Elemental based):
A fire monster equipped with a '+5% fire attack' item will: A) Do 5% more damage against fire, and B) Do 5% more damage with fire-based attacks.
Items will NOT boost each other's bonuses, so a +40% attack and a +50% attack will result in 190% total damage, not 210%.
For example: A +40% Physical ATK item will: A) Add 40% to all Physical damage they do, and B) Add another 20% to all damage against Physical based monsters on top of that for a total of +60% to the damage multiplier ( DamageMultiplierIcon ).
For Poison it is not the multiplier, but the roll, that is affected:
For example: Rolling a 4 on a poison die with 200% Poison ATK means your "DICE TOTALS: POISON:" will be equal to 12 instead of 4. At least two things are true when it comes to poison attacks: A) The damage multiplier can NEVER effect poison damage, and B) The dice total for poison will always round up (e.g. rolling a 2 on a poison die with an item that boost Poison ATK by +1% will result in a 3).