Per https://jira.hl7.org/browse/FHIR-20802, there's a question about whether itemPopulationContext should be able to be used without initialExpression to populate an element.  Sometimes, having the extra extension seems heavy/unnecessary.  However, if we do that, we'd need to know the rules for when populationContext populates by itself and when you need initialExpression.

(Might also be good to understand what the difference is between populationContext and variable...)