Builds within a few points of each other count as one, and the rows are ranked by how often people shared, saved or opened them. 7 complete builds from before the trees changed are left out, since the game would not take them now.
talent tag = an ability a talent grants. It shows in your book once you take the talent.
Small numbers under a spell are the level each rank is learned at.
Numbers are the beta client's at level 60 with no gear on, so your in-game tooltip will read a bit higher. Show all ranks works like the checkbox in the game's book. The General tab is kept by hand; each tooltip there says where it was read.
One race's in Classic, every Priest learns them now. Hover a spell to read it.
The challenges are the client's achievement rows that reward a Legacy point, build 1.60.1.70170. Blizzard's Legacy video shows the same list.
Source: the Forever beta client, build 1.60.1.70170. Positions, prerequisites, point gates and every rank's text are the client's; the rules will be checked in game.