This table explains the data between TradeRunner and BigCommerce

Omnivore Data   BigCommerce Data
SKU    This is the BigCommerce product ID - a number generated by BigCommerce when you create a product
Click here to find how to find your Product ID in BigCommerce
Retailer Reference    This is a unique value constructed by TradeRunner from Bigcommerce data to provide a unique value for each product variant in case of multi-variant products

Please Note:

This is important information to note when creating Product Groups in Trade Runner.  Product Groups require the SKU we have in TradeRunner NOT the retailer sku you may have in BigCommerce.

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