For example, on nuget.org there are these preview packages published for System.Text.Json:
6.0.0-rc.2.21480.5
6.0.0-rc.1.21451.13
6.0.0-preview.7.21377.19
6.0.0-preview.6.21352.12
6.0.0-preview.5.21301.5
6.0.0-preview.4.21253.7
6.0.0-preview.3.21201.4
6.0.0-preview.2.21154.6
6.0.0-preview.1.21102.12
I get the 'preview.n' and 'rc.n' parts. I believe these relate to the wider .NET releases.
I wondered if the third number is number of days since some baseline date, but that doesn't seem to be consistent (based on subtracting that number from publication date you get different baseline dates); if it was then I would have guessed the fourth number to be the build number within a given day.
I ask because I am interested in understanding what package versioning strategies are in use in the wild, and whether there is anything to be learned from this scheme in use by Microsoft.