Assignee not exposed in activity import
The record_type_id allows for source and target but not assignee? Wondering the history and reasoning behind this? https://forum.civicrm.org/index.php?topic=29394.0
It is possible to use the API CSV Import extension instead which previously may have hit issues with this: https://civicrm.stackexchange.com/questions/42834/issue-with-api-csv-importer-and-activity-imports/45547#45547.