How this tool works, in plain terms.
Enter a character, and this tool shows you which gear slots are not the item your guide recommends for your spec, and where the right one drops.
Sign in with Battle.net. This tool is for Unstable guild members only — if your account has no characters in the guild, you will see a page saying so, with a link to sign out and try a different account.
A gap is a slot where you are not wearing the guide's pick for your spec. Already right means you already are.
Every slot the guide names for your spec falls into exactly one of three buckets: a gap, already right, or above what your current key level's dungeon chest can give you (see Key level, below) — a real state for a well-geared character, not a mistake. The "Already right" tile's second line spells out all three counts, and they always add up to the total number of slots the guide names.
The key level you pick decides the item level a dungeon's end-of-run chest would give you at that level, and that decides what counts as an upgrade. Raise it and an "already right" slot can become a gap; lower it and a gap can become a slot above the ceiling instead.
On (the default) counts raid drops as part of what a gap can be. Off shows Mythic+ only — useful if you do not raid, since a raid-only upgrade you will never go and get is not a gap worth chasing.
If your roster entry says you do not raid, this toggle does not appear at all for that character, and the page always stays Mythic+-only.
By dungeon is what to queue: gaps grouped by where the right item drops. By slot is how you stand: every slot the guide names, including the ones already right, in the order you wear them.
Add your characters from the manager. For each one, you set: spec, key level, whether they raid, whether they are parked (skipped entirely, for an alt you are not currently playing), and which slots you already expect to replace soon, so this tool stops naming them.
What you declare always wins. If Blizzard's own data disagrees — a different spec played last, say — you will see a note about it, but nothing changes until you say so.
Every "fetched" timestamp says when that gear was actually read from Blizzard. Pressing "Look up" fetches right then; a character nobody has looked up before is fetched automatically the first time, however you reach them. Roster characters are also refreshed on a schedule in the background, so a roster page can show gear from a little while ago — the timestamp always says exactly how long.
Gear comes from Blizzard's own game data. The guide's picks come from Icy Veins. Every page names both, and when they were last read, in the footer.