The Backup and Transfer will retain all of your member data and posts, and maintain the associations between them. By using the Import/Export, you will have seemingly random member numbers, and topic assignments.
Charles Capps post at UBBDev
The importer assigns completely random category/forum/post/topic/member IDs. Importing an export will result in completely different user and post numbers. This is not an effective backup tool for that reason.
Source:
https://www.ubbdev.com/ubb/ultimatebb.php/topic/8/3682.html#000000
