What do the "crossed out" fields mean?

The crossed out fields are elements that have no data, as they haven't been mapped in the import. If a crossed out field is combined with a given element, then the value of this element will not be sent to this field even though mapping has been performed, because our system does not have this bit of data. You can map the crossed out fields (1), since you may decide to add them to your import later; then the data will be automatically updated in your export