Set Bonus Skills

Returning from Monster Hunter: World are Set Bonus Skills, which are only applied when you have multiple pieces of the same Armor Set equipped. Two pieces are needed to enable the armor set’s related skill, while four pieces will provide an enhanced version. It’s a win-win if you want to slay monsters with a specific aesthetic. If you prefer to have more variety in your looks, you’ll need to make use of the transmog system.
I’ll also add that this is only available with specific armor sets. They need to be crafted from large monsters and even then, only 14 out of the 29 large monsters in the base game have armor with Set Bonus Skills. You’ll encounter a majority of them in chapter three and onward in High Rank.