Damage reduction in WoW is multiplicative, so you can never reach 100% with stacking abilities.
For example:
As a rogue, you’re playing 2s with a discipline priest and you forgot to put on any off your gear.
The opponent hits you with a 100k hammer of wrath and you die.
If you had your gear on, you would have 10% damage reduction from versatility.
You would instead take 100k * 0.9 (90% damage taken, 10% damage reduction) = 90k damage.
Now let’s say your priest uses pain suppression, you now have 10% damage reduction from versatility and 40% damage reduction from PS. Additive that would equal 50%. Multiplicative it equals 46%.
Damage taken = 90% * 60% or 0.9 * 0.6 = 0.54. 54% damage taken means 46% damage reduced.
Let’s add elusiveness feint which is 30% more damage reduction. Now you have:
90% damage taken (vers) * 60% damage taken (PS) * 70% damage taken (feint)
0.9 * 0.6 * 0.7 = 37.8% damage taken or 62.2% damage reduction.
Hope it makes sense. Just remember it’s multiplicative and not additive.