Weapon Expertise: These sources are additive to each other. Examples of this include: weapon specialization perks, ammunition bonus from bows and crossbows, Malacath scaling boon, Dibella free boon, the armor penetration enchantment, and so on.
For weapon expertise, the effects are penalized for normal attacks at a 50% rate. For example, if you have 20 weapon expertise, then you will penetrate 20% of an enemy's armor with power attacks, but only 10% with normal attacks. Ranged weapons always have full penetration, but rely on high-quality ammunition.
The cap for armor penetration in this way is 80%. So even with 100 weapon expertise, you will only penetrate 80% of the enemy's armor with a power attack. However, normal attacks can still benefit from going over the cap due to the penalty.
To find out your weapon expertise, you can find it in the Requiem Skills Menu MCM.
"Ignore Armor" Effects: These are a separate multiplier from above and none of them are combined. Examples include dragonbone weapon material bonus, Namira boon, bound weapon perks, and so on.
There is no cap for this since they're all unique modifiers.
Example Scenario: A Khajiit attacks an 800 armor enemy with a dragonbone mace using a power attack. The khajiit has all weapon specialization perks for the mace, so 50 weapon expertise.
800 armor base * 0.9 (khajiit racial) * 0.75 (dragonbone material) * 0.5 (weapon expertise) = 270 armor rating for damage calculations.
If instead the Khajiit uses a normal attack....
800 armor base * 0.9 * 0.75 * 0.75 = 405