Purging archived files
When an item with an underlying file — i.e. not a BLOON doc or URL — is erased from limbo, depending on your settings, the file itself might remain on your hard drive instead of being deleted right away. Even if an item has been erased from limbo by another member of the bloon, the underlying file may still be accessible on your device for some period of time. This is archiving.
Whether underlying files are archived or purged when items are erased from limbo is a setting in itself. See "Purging vs archiving underlying files when items in limbo are erased."
Archiving also takes place when a file synced across multiple devices is modified. On the devices other than the one where the file was modified, the older version of the file will be archived; the edited version will be downloaded.
See "Archiving" for more of the basics on ... archiving.
What happens to archived files is up to you. They can be purged automatically after a certain number of days, or remain on your hard drive unless you purge them manually. This setting is device-specific. If you have multiple devices associated with the same account, each one will have its own independent setting for whether and when archived files are automatically purged.
By default, archived files are automatically purged after 90 days.
Manually purging archived files
If you select "Archived files should not be purged automatically", archived files will remain on your hard drive indefinitely.
Whenever you want to selectively purge archived files — to erase bulky video files, for instance — you can do so by wading into the archived file pool.
Automatically purging archived files
If you choose the option specifying that "[a]rchived files should be purged after ____ days", archived files will be permanently destroyed after that number of days, upon which the corresponding space on your hard drive is freed up. The default number of days for this option is 90 days on fresh installs of BLOON. You can adjust this number as desired.
BLOON scans for files due for purging when it starts, and every 12 hours subsequently.
Archiving & backup servers
You can configure a machine as a backup server — where copies of files are permanently kept — by opting for (1) underlying files to be archived when items are erased from limbo ("Files deleted from limbo should be locally archived") and (2) manual purging (i.e. no automatic purging).