In the 278 (Health Care Services Review — Request for Review and Response, please explain the difference between the UM02 (Certification Type Code) code 4 (Extension) and code S (Revised). Both codes appear to have basically the same definition based upon the description in the Front Matter.
This issue is explicitly addressed in guide 005010X217. The note on code 4 (Extension) reads "Indicates that this is an extension request to a prior approved service." The note on code S reads "Use if the requester is revising the specifics of a certification for which services have not been rendered."
Code S can only be used when revising the certification BEFORE services are rendered, and supports changing the services requested. Code 4 applies also after some services have been rendered, and only supports extending the prior approved service, not changing to a different service.
For example, an extension is when an authorization has been approved for 5 visits and it is being requested to extend the authorization to 10. A revision is when you want to change the procedure that was previous authorized, but the service has not been rendered. This will be clarified in the next version of the guide.