Required headers in exact order:
ClientID, PO Number, SKU, Style Code, Product Name, Size, Colour, SSCC, EAN, Total Item Quantity, SSCC Quantity, Carton UOM Quantity
Each row must contain exactly one identifier: either SSCC or EAN.
EAN rows derive carton_ean_count from Total Item Quantity / Carton UOM Quantity. Missing UOM or rounded-up counts upload with a warning.