SafetyNet is a feature unique to CCC that aims to protect data on your destination volume. Suppose, for example, that you have three volumes: Macintosh HD, Backup, and Photos. If you created a backup task and accidentally selected the Photos volume as your destination, some copying applications would simply erase the destination or delete the files on that volume, with no recourse! With SafetyNet enabled, CCC benevolently retains those items on the destination, but cordons them off to a separate folder so you can recover them later if necessary.
On a snapshot-enabled volume, the SafetyNet is now implemented as a pre-flight snapshot. Before CCC makes any changes to the destination, it will create a "SafetyNet Snapshot" of the destination. Then the task will proceed in the normal manner, copying files from the source to the destination. If you later realize that you had configured the task with the wrong destination, or that you had placed files on the destination volume and they're missing after running your backup task, you can restore those items to the destination from the SafetyNet Snapshot.
At the end of the backup task, CCC will create a second snapshot; a "Backup Snapshot". This second snapshot represents the state of the source for the current backup event. If you ever wanted to restore data back to the original source or to a replacement disk (e.g. because the source disk failed), you would use a Backup Snapshot to restore that data. This is a very important point: you generally will never use a SafetyNet snapshot to restore data back to the original source. SafetyNet Snapshots are used to restore files that were errantly deleted or modified on the destination.
Summarizing, keep these two points in mind:
SafetyNet Snapshots allow you to recover files on the destination that were unrelated to your backup task
Backup Snapshots give you point-in-time restores of the data from your source volume
Do I need SafetyNet?
You typically do not need SafetyNet to be enabled on your backup tasks. SafetyNet snapshots offer protection from configuration mistakes, e.g. selecting the wrong destination, or using the destination to store files that are not related to the backup task. As long as your destination is dedicated to the backup task, SafetyNet is an unnecessary layer of snapshot redundancy.
SafetyNet is a safety mechanism, it's not a strategy for retaining other stuff on your backup volume
Your backup volume should be dedicated to your backup task. If you want to take advantage of some extra space on your backup disk, we recommend that you add a volume to that disk specifically for storing the other data. That other volume will be outside of CCC's purview, thus protected from any unintentional alterations. Open Disk Utility and select your backup disk, then choose Add an APFS Volume... from the Edit menu to add a volume to your backup disk.