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. 840 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.
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.