The required id attribute is the QIF id of the statistical studyresults, used for referencing.
Source
<xs:element name="CapabilityStudyResults" type="CapabilityStudyResultsType" substitutionGroup="StatisticalStudyResults"><xs:annotation><xs:documentation>The global CapabilityStudyResults element gives information about the results of a capability statistical study.</xs:documentation></xs:annotation><xs:key name="CapabilitySubgroupIdKey"><xs:annotation><xs:documentation>The CapabilitySubgroupIdKey collects all QIF ids of all Subgroups in CharacteristicStats of a capability study.</xs:documentation></xs:annotation><xs:selector xpath="t:CharacteristicsStats/t:*/t:Subgroups/t:Subgroup"/><xs:field xpath="@id"/></xs:key><xs:keyref name="CapabilitySubgroupIdKeyref" refer="CapabilitySubgroupIdKey"><xs:annotation><xs:documentation>The CapabilitySubgroupIdKeyref requires that a SubgroupId in a StudyIssue in CharacteristicStats be the id of a Subgroup in the same capability study.</xs:documentation></xs:annotation><xs:selector xpath="t:CharacteristicsStats/t:*/t:StudyIssues/t:StudyIssue /t:SubgroupIds/t:Id"/><xs:field xpath="."/></xs:keyref></xs:element>