SKUs are internal. Each company designs its own SKU scheme to suit its systems, categories and reporting, so the same product usually has a different SKU at the manufacturer, the distributor and the retailer. A shirt in three sizes and two colors is typically six SKUs, because each combination is stocked and counted separately.
That is the key difference in SKU vs GTIN. A GTIN is a global identifier set by the brand owner and shared by everyone who handles the product. A SKU is private to one company. In practice, companies need a cross-reference between their own SKU, the supplier’s part number and the GTIN, which is built and maintained during new item setup.
When that cross-reference is wrong or missing, problems spread across the order flow. A purchase order (EDI 850) can reference an item the supplier doesn’t recognize, an ASN can be rejected, and invoice lines can fail to match. Keeping the mapping accurate is a core product-data task, often supported by PIM and MDM systems.