Purpose of "Retention Days"?

[wilcoxiv]

I couldn't find any documentation on this feature. What exactly is being retained in which database? I am guessing anything scanned after x days would need to be re-sync'd?

[jfranki]

I would like to know this too...

[Eddict]

from the source:

        public string Description => "If retention days > 0 then this will remove the old data to keep information flowing quickly";