Board Thread:Questions and Answers/@comment-5149755-20150409001207/@comment-27384850-20160603155001

We now have info about when Skill Slots appear, courtesy of Progressive v2. Players start with two skill slots, with new ones appearing at levels six, twelve, twenty, and then every ten levels afterwards.

As for equipped skills v. those on the skill list, it's as Rich Richie says. Those on the list are available for the player to use, while only the ones actually equipped can be used/leveled.