A vertical split uses vertical cut lines. They run top to bottom through the picture, and the pieces sit side by side. Every panel keeps the full height of the source and takes a share of its width.
The phrase confuses people because it can be read either way, so it is worth being blunt about it. Vertical lines, horizontal arrangement. If you want stacked pieces cut by lines that run left to right, that is the horizontal splitter.
Columns go from 2 up to 20. A 3000 × 1000 panorama into three columns gives three 1000 × 1000 px panels. The same panorama into five gives five 600 × 1000 px panels, narrow enough that a carousel is really the only place they work.
Seams are draggable, which matters more on vertical cuts than most people expect. Faces, doorways and vertical lines in architecture all sit where an even split wants to cut. Move a seam a few percent and the panels usually improve. The preview shows all the panels at once so you can balance them in one pass.
The rows axis is locked at one so this stays a pure vertical cut. Everything runs on a canvas in your browser, so nothing is uploaded and there is no queue, no watermark and no limit on how many images you split.