How does archiving behave when enabled on the settings in my account?

You can set tFlow account to archive old orders by enabling that functionality under System Settings > Archiving window in your account. This is documented in the articles tFLOW Archival Function and tFLOW Archival Function to Amazon S3.
Archiving is done by a scheduled task. By default, that task is executed at 12:00AM every Saturday based on the server time of your tFlow instance.
There are three possible settings for determining which orders to archive:
1. Approve Only Orders 'Completed'
This is to archive unmodified jobs in a workflow's terminal states (i.e. Sent to Production, Cancelled, etcetera). Jobs are getting archived only when all jobs in given order should be archived or are already archived. This archives only whole orders, not individual jobs.
2. Bluemedia Schema (or Approved)
This is to archive unmodified jobs which are approved or cancelled. Like with the first option above, only whole orders are getting archived. Note that "Approved" jobs means not only jobs in "Approved" state, but also in any state *after* approval, including "Sent to Production" and other states depending on the workflow.
3. Approve All Independently from the Status
This will archive unmodified jobs in any state. In this mode, jobs can be archived separately, and does not depend on other jobs statuses in the same order. A job is considered "unmodified" if it has not been changed for the specified period of time. Due to internal limitations, changing user assignments, editing profile variable values, and managing attached support files are NOT counted as job modifications.

A job is considered "unmodified" if it has not changed (status update, new revision uploaded, job re-processed, etc..) for the specified period of time (Start with months ago).