In most cases, all calls support a MessageID element in the request and a CorrelationID element in the response. If you pass a message ID in a request, we will return the same value in CorrelationID in the response. You can use this for tracking that a response is returned for every request and to match particular responses to particular requests. If you do not pass MessageID in the request, CorrelationID is not returned.
Note: GetCategories, GetAttributesCS, GetCategory2CS, GetAttributesXSL, GetProductFinder, GetProductFinderXSL, and GetProductSearchPage are designed to retrieve very large sets of metadata that change once a day or less often. To improve performance, these calls return cached responses when you request all available data (with no filters). When this occurs, the MessageID and CorrelationID fields aren't applicable. However, if you specify an input filter to reduce the amount of data returned, the calls retrieve the latest data (not cached). When this occurs, MessageID and CorrelationID are applicable.