A file/folder gets marked as "archived" when it is deleted from the local device.
Let's say you have a document called "Invoices" backed up to ElephantDrive from your MAC. Then, you delete the Invoices file from you MAC. Once Invoices file gets deleted from your MAC, ElephantDrive will recognize that the file no longer exists on the local device and will archive the file.
A file will also be archived if you move the file to a different folder or if you change the name of the file.
Change Archiving Settings:
Archived files can be retained for any number of days. To change how long to retain archived files for, see "Setting Archiving Rules".