The 9 cut is the same nine tiles as a grid post, with a different emphasis. It is a photo trend more than a design exercise: one good picture, cut into nine, posted as a set, usually with a short caption on each tile.
Which means the source matters more than the layout does. A picture with one clear subject and space around it survives being cut into nine. A busy one turns into nine unreadable fragments. Before you export, look at the preview as nine separate squares rather than as one picture, because that is how most people will see it in a feed.
The crop is square, so a rectangular photo loses something. You choose where the square sits: centre, or pulled toward a subject. Then the nine cuts are equal, giving nine tiles from the cropped square.
Posting is the reverse of reading. Bottom right first, top left last, because the newest post takes the top left corner. The tool numbers the files in that order so you can upload straight down the list.
There is one wrinkle worth knowing. Instagram now crops profile thumbnails to 3:4 portrait, so each square tile shows with its top and bottom trimmed in the grid view. Keep the subject in the middle band of each tile and the cut still reads.
Nothing is uploaded.