Build the intended seam
Use the same thread, stitch length and seam allowance planned for the sofa. Sew samples in both principal directions and mark them so the workshop can identify the one under seat tension.
Ask for the matching evidence
ACT lists seam slippage for woven upholstery. Request results for the offered construction and note the test direction. If the mock-up shows movement, review the seam design before committing to the lot.