QI Core section 1.12 addresses Must Support requirements and references US Core requirements via a link. That link currently goes to the full US Core page General Guidance . Will change that link to the specific section of the US page 2.2 Must Support . This guidance specifically states, "The Profile elements consist of both Mandatory and Must Support elements. Mandatory elements are elements with an minimum cardinality of 1 (min=1). The base FHIR Must Support guidance requires specifications to define exactly the support expected for profile elements labeled Must Support. The sections below illustrate how these elements are displayed and define the rules for interpreting profile elements and subelements labeled Mandatory and Must Support for requesters and responders." To provide further support, will assure that all elements with cardinality of 1.. have a Must Support flag, and further that only subelements with cardinality of 1.. that are part of Must Support elements will also be tagged as Must Support.