How to interpret the results of DDPEval.exe?

I have read the documentation for DDPEval.exe and various blogs and so on and cannot find any information on how to interpret the results.

I ran the tool on a drive on one of our servers and found the results confusing, particularly with regards to "Optimized files" and "compression". To try and get some more understanding I ran the tool again on a specific folder only, where is there no NTFS compression in place (contents are SQL Server backups). For brevity I will post only the results from this second test:

Evaluated folder size: 69.07 GB
Files in evaluated folder: 6

Processed files: 6
Processed files size: 69.07 GB
Optimized files size: 15.61 GB
Space savings: 53.45 GB
Space savings percent: 77

Optimized files size (no compression): 69.04 GB
Space savings (no compression): 26.93 MB
Space savings percent (no compression): 0

What do these numbers mean and is the tool telling me I can save 53.45GB or 26.93MB?

You can save 53.45GB reducing space usage by 77% in case if you use both deduplication and compression.

You can save 26.93MB if only the deduplication is used.

Usually, deduplication gains are calculated using the deduplication ratio. I would recommend you using an alternative free tool called Deduplication Analyzer: https://www.starwindsoftware.com/starwind-deduplication-analyzer.

It gives you a more transparent and understandable outcome providing an industry-standard Deduplication ratio parameter.