The message body usually contains a representation of the requested resource, one or more URLs that satisfy the request, or both. In some cases, the message body should be empty, as specified in RFC 2616, Section 4.3

You can use the pub.flow:setResponse service to explicitly set the message body. See the webMethods Integration Server Built-In Services Reference for details. If you do not explicitly set the message body, the output pipeline of the top-level service will be returned to the client in the message body.

For more information about how Integration Server builds HTTP responses, refer to the section that describes how Integration Server chooses content handlers in webMethods Integration Server Administrator’s Guide.