About Upstream version choice

[罗瑶明]

Dear ubuntu-devel-discuss,
          I have questions about how to choice or decide which Upstream version to use for a certain software in Ubuntu Derived series.  
is there some guiding principle or refer documents. i.e.,why choice gcc-9.3.0 in the Focal Fossa. 
                                                                                                                                                                thanks.
                    
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