There are two completely different things people mean by splitting a photo for Instagram, and picking the wrong one wastes an afternoon.
A grid split spreads one picture across your profile as separate posts. Nine tiles fills three rows. It looks like a billboard at the top of your feed, it needs a square crop, and you have to upload the tiles backwards because the newest post takes the top left corner.
A carousel split puts the pieces inside one post that people swipe through. The pieces stay together, the order is left to right, and the shape is portrait rather than square. Up to 20 slides in a single post.
The chooser at the top of the splitter switches between them, and everything downstream changes with it: the crop, the padding, the file numbering and the guidance under the preview.
Two things to know either way. Instagram now crops profile thumbnails to 3:4 portrait, so grid tiles lose a slice off the top and bottom in the grid view. And every slide in a carousel has to share one aspect ratio, because Instagram crops mismatched slides to match the first one. Padding each slide to 1080 × 1350 px solves that in one step.
All of it runs in your browser. Your photo is never uploaded.