Sub-classes are what make characters of the same class really able to differentiate themselves from one-another. In Unearthed Arcana, WotC tries out new rules that will be coming to Dungeons & Dragons. Put the two together and you've got an Unearthed Arcana that looks into some new subclasses. That's what we've got here, with an updated version of some subclasses they've posted previously.
From the post:
Three subclasses reappear today in Unearthed Arcana, each with a new form. After studying your feedback on the Revived, Noble Genie, and Archivist subclasses, we’ve crafted new versions for your consideration: the Phantom, the Genie, and the Order of Scribes. You can find them in the PDF below.