You can equip special runes to any of your skills, including classes that have transformation skills (Scouter, Shadowhunter, Artillerist). Each skill may be equipped with one rune, and you can have a maximum of 8 runes equipped. These runes impart additional bonuses specifically when you are using that skill. Runes come from a variety of contents. They can freely be equipped and removed from their skills at no cost. Skill presets will also memorize which skill is set to use which rune.
In certain cases, you are able to obtain more than one copy of a rune if it has multiple sources. For example the Galewind Rune, you are allowed one copy of the rune from each raid tier.
In the case of the Bleed rune, Bleed does not stack from the same player (in the case of equipping multiple Bleed runes) and inflicts the same amount of damage at all rarities. Different Bleed stacks can be applied on the same target by different players.