The block of cells that a single dynamic-array formula fills, referenced with a # suffix.
Try it with AIWhen a dynamic-array formula returns multiple results, they spill into adjacent cells — that block is the spill range. Refer to the whole range with the anchor cell plus #, e.g. A2#.
=SUM(A2#) (← sums the entire spill range)It's the spill operator — A2# means "the whole range that the formula in A2 spilled into."