There is a genuine ambiguity in the phrase, so here is how this page reads it. A horizontal split uses horizontal cut lines. The lines run left to right across the picture, and the pieces come out stacked: top, middle, bottom. Columns stay at one throughout, so every strip is the full width of the source.
If you wanted the other thing, side-by-side pieces cut by vertical lines, that is the vertical splitter.
The row count is yours to set, from 2 up to 20. A 1080 × 4000 long-form graphic cut into four rows gives four 1080 × 1000 px strips. Each keeps the full width, so they stack back into the original with no horizontal seams to line up.
Strips are where draggable cut lines earn their place. Long content has natural breaks: the end of a section, the gap above a heading, the line between two chart panels. Equal rows will cut straight through a paragraph. Drag each line into a break and the strips read properly on their own. The preview shows every cut at once, so you can balance them all before exporting anything.
The columns axis is locked here so the preset stays a pure horizontal cut. Everything runs on a canvas inside the page, with nothing uploaded and no limit on how many images you put through it.