Provide the following guidance: if the allowed set of answers for a question changes after an answer has been specified (whether via population or user entry), the prior answer SHOULD be retained in the user interface until such time as the questionaire is marked as 'complete', even if invalid.  This is to minimize impact on users if they accidentally toggle elements within the form that govern the answerExpression.  However, in some cases, this will not be possible depending on the data entry controls.  Systems MAY wipe the answer on the grounds that a change to the list of possible answers might result in different answers.  If an answer becomes invalid based on evaluation of answerExpression, calculated values falling out of range, etc., the form SHALL flag the answer as invalid and prevent the QuestionnaireResponse from being marked as 'complete'. In answerExpression, note that if the answerExpression relies on content outside the QuestionnaireResponse (e.g. launch context or query results), the ability to consistently check the validity of the QuestionnaireResponse - particularly for downstream systems - may be difficult.  One potential solution is to populate internal (and potentially hidden questions) with information passed in by context or found through query such that validation of answers can be based entirely on information stored within the QuestionnaireResponse, though privacy and/or storage considerations may make this impractical in some cases. On 'hidden', highlight that hiding data from the enterer is not a solution to data privacy/security concerns as any questionnaire answer must be presumed to be potentially available to any consumer of the questionnaire in some manner. Note that it is possible that answerExpression and initialExpression can both exist.  InitialExpression runs on launch and answerExpression should run whenever the user interface needs to render the question, the user interacts with the question or during validation. Also note that answerExpresssion and calculatedExpression can both exist and are re-calculated when the source for their expressions change.  If the calculatedExpression is not valid according to the results of the current answerExpression, the QuestionnaireResponse will fail validation and be unable to be marked as 'completed' until corrected. Will also ensure that there's language that indicates that the result of initialExpression, calculatedExpression and answerExpression must all align with the 'type' of the item they appear on - e.g. can't have an expression that returns a string if the type is Choice/Coding.