"This extension contains the endpoint for payer services." isn't sufficiently clear.

Change to

"This indicates the base URL for the payer DTR interface relevant for a particular patient Coverage.  This URL will be the base for invoking the $questionnaire-package operation, $next-question operation, persisting DTR work-in-progress, etc.

The same URL might be used for multiple payers, and in some cases, a payer might use different interfaces for different plans.

If this extension is not present, the DTR system will have to infer the base URL based on the coverage information provided and internal knowledge of payer services."

Also, I think this extension should be limited to 0..1?  I don't think one Coverage can have multiple endpoints?