A file/folder gets marked as "archived" when it is deleted from your local device.
Let's say you have a document named "Invoices" backed up to the Vault. Then, you delete the "Invoices" file from your ReadyNAS device. Once the file gets deleted from your local device, the Vault will recognize that the file no longer exists on the local device and will archive the file in the Vault.
A file will also be archived if you move the file to a different folder/share or if you change the name of the file.