Amazon Merchant SKU
Amazon uses a Merchant SKU to identify a Seller's product. While the listing has an ASIN as the unique identifier, the Seller relates their product to that listing with their Merchant SKU. So, if a seller adds a listing without specifying the Merchant SKU, Amazon will auto-generate it with a random combination of numbers and letters.
It is possible to relate these random Merchant SKUs to regular product SKUs, but ideally you should try to use a Merchant SKU that will help you recognize the product. By default, SellerCloud will post the Product ID as the Merchant SKU when posting new products to Amazon.