I suggest you clarify the impact of this statement with regards to data elements with a minimum cardinality of 1. Can the resources still be sent if such a data element is missing? If not, does that preclude the sender from sending the notification bundle if a required resource is not available because a required element can't be populated? Similar clarification is suggested for section 2.1.6.2.1 as well.

Existing Wording:

In situations where information on a particular data element is not present and the reason for absence is unknown, the Sender SHALL NOT include the data elements in the resource instance as part of a $process-message operation or a query response.