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.
If a seller adds a listing without specifying the MerchantSKU, Amazon will auto-generate it with a random combination of numbers and letters. It is possible to relate these random MerchantSKUs to regular product skus but ideally you should really try to use a MerchantSKU that will help you recognize the product.
By default, SellerCloud will post the ProductID as the MerchantSKU when posting new products to Amazon.