SingleFeed sends product category data to each shopping engine based on their proprietary feed specifications. Most shopping engines have their own category taxonomy. For these engines, we send them just what they want - the category from their spec that most closely matches the product. However, a few shopping engines do not have their own category taxonomy and instead ask for your merchant category field. For these engines we send the category value that you have specified in the SingleFeed "Category" field. Below is chart that outlines which engines have their own category taxonomy and which do not. It also indicates which engines receive your merchant category field.