Background: Implementer feedback identified the need for more precise language surrounding the use of must support to describe choice and reference elements. It was determined that  "...a flag that says the server must support at least one data type in value [x] , but without specifying which data types..." would expose an implementer to the potential burden of supporting datatype it would normally not support, for example, the SampleData datatype. As a result Must Support tags for individual choice type and reference profiles were introduced to US Core in the January 2021 Ballot cycle The details of these changes can be found in the 2021 US Core Ballot Version note to balloters and change log ( http://hl7.org/fhir/us/core/2021Jan/ ). Based on a survey of what venders will Must Support these data types were identified as must support elements for Observation Lab: Quantity string CodeableConcept These type were also used in the Assessment Observation Profile as well.  In addition, the boolean type was added to this list to support the following pattern documented in the profile page: "Often the pattern for these types of observations that the Observation.code indicates a statement about findings and the Observation.value is present and “qualifies” the finding typically confirming or refuting it." Reasoning: By determining which datatypes are at a minimum must support and equally important which are not, the ambiguity surrounding implementation and testing is removed. We find the commenters suggestion to "1..1 without a MS tag" not persuasive because it reintroduce that ambiguity. Proposed Changes: There is no action needed by US Core on this issue.