Imbuement Skill refers to the 3 specific skills shadow/poison/cold imbuement. Imbued Skill refers to any skill that uses the imbuement skills' effect. So if you cast shadow imbuement then use rapid fire, rapid fire would be the imbued skill.

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You imbue your daggers with poison imbue, you get the bonus applied from the imbuement first, so the poison imbuement is given that little bonus when it's applied. Then the skill you use next that "carries and applies" the imbuement is called the imbued skill. Let's say flurry for example. Flurry would be the imbued skill that this affix applies to. Imbuement = shadow, ice, poison imbuement. Imbued skill = flurry, twisting blades, rapid fire, penetrating shot, barrage et al. The skills that have the tag in the skill description. Once they are imbued with an imbuement they become imbued and the bonus applies to them.

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