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. You can also set the message body of an HTTP request by adding a variable named $httpResponse that references the pub.flow:httpResponse document type to the flow service pipeline. For more information on this document type, see webMethods Integration Server Built-In Services Reference. 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, see webMethods Integration Server Administrator’s Guide.