Certain elements in the profiles defined in this implementation guide are marked as  Must Support . This flag is used to indicate that the element plays a critical role in defining and sharing quality measures, and implementations SHALL understand and process the element.  In addition, because this specification makes use of data implementation guides (e.g. US Core, QI-Core), the implications of the Must Support flag for profiles used from those implementation guides must be considered.  For more information, see the definition of  Must Support  in the base FHIR specification.  Add a STU Note in the IG that says Must Support guidance here requires additional clarifications, we are seeking implementer feedback on what type of guidance would be most useful.