There are 2 types of armorings that carry set bonuses to whatever armor you put them in. Campaign armoring drops in hard mode explosive conflict and Dread Master armoring drops in nightmare explosive conflict and hard mode terror from beyond. You get the tokens from boss kills, turn them in to the vendor for your gear and then yank the armorings from them for use in whatever armor you choose.
One caveat is that the campaign feet token only drops off the world boss on Voss (Nightmare Pilgrim).
Those armorings are specific to a type of gear (gloves, for example) and can only be used in that type of gear. They also cannot be learned by crafters.
So, if you want to have your pve set bonuses and wear whatever gear you want, you have to be able to at least clear hard mode explosive conflict and farm the armorings.
not true. You can buy slot-specific campaign armorings with set bonus from one of the daily vendors for black hole comms now. Yes those comms are better spent on black hole gear itself, but it is still possible to get them without even touching an ops, given you have the time to do so.