DICOM PS3.18 2019a - Web Services
6.9.7.3.2 Response Headers

If the CreateSubscriptionrequest was accepted but the deletion lock was not, the response message shall include the following HTTP Warning header field:

If the request was rejected with an HTTP 403 status code because Filtered Global Subscription is not supported, the response message shall include the following HTTP Warning header field:

DICOM PS3.18 2019a - Web Services