So let me say once and for all -- the time has come for us all to stop holding backup tapes for years at a time and pretending they're an archive. While old DLT7000, or even worse, DDS tapes at Iron Mountain may meet the legal definition of retention they don't make a useful archive.
The existential difference between backup repositories and archives isn't the media they use or the hardware they're built on but their purpose. As a writer I find this clear in the language we use to describe the process of getting data from each type of data store.
We make backups in order to restore things like servers, databases, file systems, mailboxes or even individual files or email messages to their previous condition should they be lost, damaged, deleted or corrupted. Restores, in general, return things to their original place and condition so they can be used for their original purpose.
Archives on the other hand exist so data can be retrieved. Once retrieved that data is usually used in a different way than when it was originally created. Emails can be restored to be answered or acted on, or they can be retrieved to settle an argument, legal or otherwise.