A draft is drawn on squared paper. One square across is one warp end; one square down is one pick of weft. Filled in means that end floats over.
Get one square wrong and a single end is lifting when it should be sinking, on a single pick. On its own that is one point. But the repeat runs the length of the piece, so that point recurs — and what you get is a broken line running down the entire bolt.
Checking the file before it goes to the loom catches structurally illegal drafts — an end floating over twenty picks and snagging, say. It cannot catch “legal, but not what you meant”. That only shows up in cloth.
Which is why every new figure gets a fifty-centimetre sample first. Fifty centimetres covers at least two repeats, and anything that repeats will declare itself.
The interactive draft on the home page runs the real rule: drawdown[pick p][end w] = tieup[treadling[p]][threading[w]]. Move a few marks in the threading and watch the cloth rewrite itself. What one wrong square costs is a click away.
Bring a swatch, a drawing, or just a sentence — "it should drape without going soft" is enough to start. We work backwards from yarn count. Minimum run 30 metres.