https://jira.atlassian.com/browse/JRA-39749
As quoted from the above ticket regarding 2 types of CSV import

Yes the both importers are looking similar, but in the background they are completly different and that's intended - we coudn't give non-admins an exact copy of CSV Importer.

CSV Importer for Administrators (from "External System Import" menu).
It was designed as a migration tool - it takes the data from file and does everything in it's power to fit them in JIRA. So without any additional question it creates resolutions/priorities/users/etc. and does all transition/status work in silence to allow user to create issues as much similar to what he/she has in the external issue tracker as possible.
Bulk Create for non-admins (from "Issues" top menu)
It looks similar, but it's 100% secure solution for creating issues, it checks all of the necessary permissions and allows only to create issues with "Open" status. It doen't have all of functionality of the 1. importer and it never will have all of them, but we'll slowly add all missing features that we believe that are safe to be implemented.

As both imports are intended to be different, there should be a documentation for both of those. There is already a documentation about the External System Import found here but there isn't any documentation about the newer CSV import