How does slash/blunt damage work?

Solution 1:

There are three different types of attack, each making the most use of a different damage type. Additionally, each piece of armour has three different armour ratings, to counter those damage types.
Even though the linked video shows a mace for demonstrating the blunt damage and a sword for both slash and stab attacks, the point is that each damage type has its ideal use:

• Blunt damage – Strong against “harder” armor types.
• Slash damage – Strong against “softer” armor types.
• Stab damage – Strong against chain mails and the like.
  ^source

Nonetheless, although yielding several threads, your question of whether the different attacks exclusively deal their respective damage types remains unanswered.
Here are some interpretations (respectively):

• I guess slash or blunt damage applies depending on what kind of armor your opponent is wearing.

• Stab damage only applies to stab attacks and cut damage only applies to slashing attacks. Blunt damage applies to attacking with the blunt side of the weapon - on a sword it's the guard/pommel.

• I don't know, but I would assume a higher blunt damage deals more armour damage or only comes into play when doing a thrust.

• I’m pretty sure that the damage numbers do not stack. It just means that is the specific damage type it will do with that type of strike.

  Like Warhorse's video linked above, some comments, like this one, seem to suggest there are in fact three different attack types.

The most thorough comment I found is this:

• The "Slash" and "Thrust" through left- and right-clicks describe the action of the move, not the property of the attack. "Slash" means to swing the weapon towards an opponent, while "Thrust" means to poke at them with the tip of one's weapon.

  With a mace, both right- and left-clicks will primarily cause blunt damage (with certain kinds of maces having additional thrust/slash damage).
  With the axe, a Slash move would inflict both slash and blunt damage (slash due to the axe blade and blunt due to the weight/momentum of the heavy head). A Thrust on the other hand would primarily cause thrust damage due to the point of the axehead or spike hitting the opponent, with a bit of blunt damage factored in due to the handle.
  ^{source, edited}

  And further down that thread:

  My assumption would be both slash and blunt damage is applied when you hit, mitigated by the armor's respective resistance to their property. This means that plate would mitigate most of the slash damage while the cloth underneath would blunt... pun intended... the blunt damage dealt by the axehead. Main reason why it's so effective to have layers of armor to all parts of the body.

• Here is a nice comprehensive video that talks in-depth about the attack/defense mechanism, based on the OP's understanding (noteworthy is the part from 1:22 until 2:52, where he talks about this specific issue). He thinks a single attack can convey a mix of damage types.

Until the underlying calculations are published or found and explained, it seems this is the best answer you will get (as in: an inconclusive one - not my answer in particular).
In the end, I would proffer, it doesn't matter that much, since you need to choose your weapon based on the armour of your adversary - and there are weapons that work well overall (like the spiked War Axe).