BXX.zip ("XX" represents the two letter publication code) is the backup file created in the Update program before running Update.
AXX.zip is the the backup file created in the Update program after an Update has completed. Both the before and after Update backups are located in a folder called XXupdate.bak which resides in the publication folder. Extract files from either BXX.zip or AXX.zip depending on whether you want to restore prior to, or immediately after the last Update. Remember that each successive backup prior to (and after) an Update will overwite the BXX.zip and AXX.zip files. If you want to save these for historical reference, you must rename them.
The primary purpose of the “built-in” backup feature in Update is satisfy requirements by various audit agencies and is meant as a supplement to normal backup procedures by your IT department. The “built-in” backup feature in Update which creates BXX.zip (and AXX.zip) includes only files that will change as a result of the Update itself, it does not backup all First Edition files such as those for publication setup, system options, tables, etc.
The First Edition User’s Guide, Chapter 21 (Backup) on page 474 specifically states that “the backup program is not a substitute for your normal daily backups".
It is highly recommended that you define a regular backup schedule to perform complete backups of the entire data folder. In the event a backup must be restored, there will be no question as to origin, date of backup, or contents. See also Data Backup Guidelines in the Lynx Media Knowledge Base.