I am not sure if this is a clarification or a correction. The itemContext page says, "Specifies a query that identifies the resource (or set of resources for a repeating item) that should be used to populate this Questionnaire or Questionnaire.item on initial population OR to be extracted from this item once the QuestionnaireResponse is complete".

Note the "OR" I emphasized. That implies that if one could somehow use this extension only for initial population or only for extraction. If so, there does not seem to be a way indicate one's preference, and perhaps this should be split into two extensions, or maybe just changed to be used for extraction, since initialExpression can be used for prepopulation. If not, then the OR should be changed to an AND.