This table explains the data between Omnivore and Shopify

Omnivore Data   Shopify DataMarketplace Data
SKU    This is the Shopify product ID - a number generated by Shopify when you create a productSKU 
Retailer Reference    This is the Shopify Variant ID - a number generated by Shopify when you create a product
MPN    This is the Retailer SKU if available, if there is no SKU value in Shopify, this is the Product/Variant ID from ShopifyMPN field in Omnivore
From the field labelled Barcode (ISBN, UPC, GTIN, etc.) in ShopifyUPC field in Omnivore
BrandFrom the field labelled Vendor in ShopifyBrand in Omnivore

Please Note:

This is important information to note when creating Product Groups in Omnivore.  Product Groups require the SKU we have in Omnivore NOT the product SKU you may have used in 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" 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 Omnivore.  This Omnivore SKU cannot be edited to be your Shopify Product SKU and it must be used when creating groups, promotions etc.