A close is the moment an AI visibility index locks a category's rankings in. It is the fixed, dated reference point that all movement is measured against.
Markets close on a fixed bell; AI rankings close on a measurement. Once a category's recommendation queries are recorded and scored across every model, the standings post as that category's close, carrying the date it was taken. The next run measures against it: every ↑ and ↓ on the board means "versus last close".
A fixed close keeps the record honest. Without one, a ranking can be quietly re-shuffled at any time and "movement" becomes unfalsifiable. With one, every position is a dated, citable fact.