A Shopify variant can now hold up to twenty barcodes. The first remains the one used when a feed or app expects a single code, while the others can help identify the variant at point of sale. Source officielle.
A practical check
Export a variant sample and check barcode order, declared type, packaging, and the visible product. Do not label an internal code as a GTIN.
A supplier prints a UPC on the box while the retailer uses an EAN shelf label. Both can identify one variant; the selected first code still matters for channels that read only one.
What the evidence supports
An extra code does not fix a wrong variant, inaccurate stock, or bad feed mapping. Test the channel before a bulk import. Date the check and retain the source documentation so that later changes can be verified.