This table explains the data between eBaylink and Shopify.

eBaylink Data   Shopify DataeBay Data
SKU    This is the Shopify product ID - a number generated by Shopify when you create a product.SKU 
Retailer Reference    This is the Shopify Variant ID - a number generated by Shopify when you create a product.
MPN    This is the Variant SKU from Shopify.MPN field in eBay
From the field labelled Barcode (ISBN, UPC, GTIN, etc.) in Shopify.UPC field in eBay
BrandFrom the field labelled Vendor in Shopify.Brand item Specific in eBay

Please Note:

This is important information to note when creating Product Groups in eBay LINK.  Product Groups require the SKU we have in eBay LINK.  Not the sku you may have used on Shopify.

The actual field in Shopify called SKU is not a mandatory field nor a required field, so it is not a reliable field in Shopify.  Our App must have a SKU as part of the eBay listing requirement and because we cannot rely on Shopify providing that data via the SKU field we use the Shopify Product ID to create a SKU in eBay LINK.  This eBay LINK SKU cannot be edited and it must be used when creating groups, promotions etc.