Import new data plus associated sub-form data

Import new data plus associated sub-form data

For importing data there is a checkbox - "Do you want to import subform data".   It appears that this option does not allow for sub-form records to be imported against a single parent record, instead the import routine imports the sub-form record data against new parents.   Can someone confirm this??  If so it is not worth having this subform import checkbox unless there is only one sub-record associated with each parent which really defeats the purpose of having subforms..?