An input lookup basically takes input as the name suggests. It is used to search the contents of a lookup table. For example, it would take the product price, and product name as input and then match it with an internal field like a product id or an item id. Whereas an output lookup is used to write fields in search results to a static lookup table file or generate output from an existing field list. Basically, input lookup is used to enrich the data, and output lookup is used to build their information.