Remote Backup Between Two PR4100s Copy/Synchronize?

My Cloud OS 3 Personal & Network Attached Storage My Cloud Pro Series
1

I have two PR4100s on the same network and want to use one to back up the other.

When setting up this up, I have the choice of Copy or Synchronize.

If I use Copy, does that mean it copies every single file every time I run the job?

If I use Synchronize, does that mean it copies only new or changed files and deletes/renames as needed?