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Interpreting package version number

I'm running Stretch and have just installed Tcl from repository.
Synaptic reports the installed version is 8.6.6+dfsg-1+b1 .
The current upstream version is 8.6.9 .

I don't understand what "+dfsg-1+b1" is telling me.
Where is that numbering scheme described?

My goal is to understand (from upstream docs) how what I have installed differs from the current upstream version.