Use much more space than your storage has

[SamiKirves]

I tried to find explanation but failed, so I have to ask this here.

How can I save much more files into my storage than complete storage size is?
Currently my disk for backups is 7TB and Explorer shows 4TB free (so 3TB used).
But just biggest server folder in backups contains total of 11TB of files. And other folders little more.

I am running VBR 10.0.0.4461 P2 and my backup disk is ReFS.
I suspect that ReFS is the key here. Is there some deduplication inside it?
Because explorer shows this much free space, this must be inside file system, not any Veeam compression.

If this is some kind of deduplication, how can I estimate future usage? Is there some tool for it?

I am extremely happy with this feature, I just want to know reason for my happiness!

[Gostev]

The feature is called advanced ReFS integration. It is a 4 years old feature which also became available for XFS on Linux in v10. The estimation is quite simple: synthetic full backups don't take any physical disk space on these file systems with Veeam.

There's no deduplication here: rather, with this approach duplication is prevented before it happens, instead of having to deal with it later. Some people at Veeam like to call this technology #nodupe

Thanks!