The inherited constraint from the parent US Core Observation Clinical Result Profile   us-core-3 error Observation.value [x] SHALL use UCUM for coded quantity units. : ofType(Quantity).system.empty() or ofType(Quantity).system = 'http://unitsofmeasure.org'   enforces UCUM as the only system that can be used in Observation.valeQuantity.code and has been present since Ver 3.1.1.  And follows the USCDI Laboratory Result Unit of Measure Data Element Applicable Vocabulary Standard.       However, it does not display in-line in the Key Elements table view as expected.    Decision:   1) will follow up with publishing to correct the rendering issue in the Key Elements view 2) will add implementer note:  "UCUM SHALL be used as a unit type for  Observation.valeQuantity.code.  This profile inherits this constraint from the US Core Observation Clinical Result Profile.    3) Regarding the comment:  Importantly, suggest appropriate coded units be used from other systems instead of UCUM's unitless strings. e.g. 'count' vs 'counts' are two strings that require interpretation whereas a single coded count from a terminology system might be more computable.    UCUM provides for the usage of {} to express custom units. Although curly braces {} in the code element are discouraged. Curly braces provide additional descriptive text in the code expression but do not contribute to the meaning of the code. 4) Regarding the comment: consider guidance for ratios to code both the numerator and denominator. update guidance in the  Using UCUM codes in the  Quantity  datatype https://hl7.org/fhir/us/core/general-guidance.html#using-ucum-codes-in-the-quantity-datatype   Using UCUM codes in the  Quantity  datatype Both the  US Core Vital Signs Profile  and  US Core Laboratory Result Observation Profile  bind the  valueQuantity  datatypes to the  UCUM  code system. A FHIR  UCUM Codes ValueSet  that defines all UCUM codes is in the FHIR specification. This guidance specifies how to represent the Quantity datatype when the correct UCUM units are missing, or the units are missing, which will likely occur in the real world.   to  Using UCUM codes in the  Quantity  datatype Both the  US Core Vital Signs Profile  and  US Core Laboratory Result Observation Profile  bind the  valueQuantity  datatypes to the  UCUM code system. Systems should also use UCUM for the optional Observation.valueRange and Observation.valueRatio datatypes (which are complex datatypes with Quantity elements).  A FHIR  UCUM Codes ValueSet  that defines all UCUM codes is in the FHIR specification. This guidance specifies how to represent the Quantity datatype when the correct UCUM units are missing, or the units are missing, which will likely occur in the real world.