The difference between a Response Modification and a Request Modification
You want to know the difference between a Response Modification and a Request Modification
The ProxySG retrieves a requested object (ex. a web page) and sends a preview of the object to the ICAP server that supports response modification. The object preview includes the HTTP request and response headers, and the first few bytes of the object. After checking those bytes, the ICAP server either asks the ProxySG to send the remainder of the object for scanning, or sends a notification to the ProxySG that the object is clean and opts out of the transaction.
The ProxySG does not retrieve the requested object, but sends the client request to a ICAP server that supports request modification. The server might then return an HTTP response to the client (for example, sports not allowed); or the client request might be modified, such as stripping a referer header, before continuing to the origin content server.