Import either succeeds, or fails (poorly - see T7701). Many requests have come in for "warnings" to be implemented.
- the user uploads an XML dump or picks a transwiki import source, and hits submit (or via the API of course);
- MediaWiki downloads the dump, processes it, and if it encounters any warning conditions, stops and returns a warning page to the user;
- the user decides whether or not to proceed;
- if they choose to go ahead, the dump is fully imported. Otherwise the dump is not imported at all;
- the stashed dump is deleted from the server.
This is similar in spirit to the uploadstash functionality.