http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-enableWhenExpression|4.0.0-ballot

An expression that returns a boolean value for whether to enable the item. If the expression does not resolve to a boolean, it is considered an error in the design of the Questionnaire. Form renderer behavior is undefined. Some tools may attempt to force the value to be a boolean (e.g. is it a non-empty collection, non-null, non-zero - if so, then true). Systems SHOULD present a warning to the user if a non-boolean valued expression result does not trigger an all-out error.