Response groups are data sets that can be returned by Amazon E-Commerce Service (ECS). They allow you to tailor your requests to return only the data you need. Each operation, such as ItemSearch or SimilarityLookup, has a list of valid response groups that can be used with it. You can find the list of valid response groups supported for an operation by viewing that operation's ResponseGroup request parameter. There you will find the list of default as well as valid values.