http://hl7.org/fhir/StructureDefinition/characteristicExpression|5.2.0

An expression that defines the criteria for group membership. This extension can only be used on a Group resource with a membership of `definitional`. When this expression is used, the Group resource cannot have any characteristic elements; this mechanism is exclusive with characteristics. The result of the expression SHALL be boolean-valued. The expression SHALL be parameterized with a `%context` variable that represents the subject being tested for membership. If evaluation of the expression results in `true`, the subject is considered a member of the group.