On Ubuntu, We could install both java source and java doc packages separately; by installing openjdk-11-doc and openjdk-11-source (for v11).

As for utilizing these in Eclipse IDE, whereas we specify the path of the source and doc in preference -> installed JRE, I just wonder what's the difference between doc and source packages when Eclipse shows a description pop-up about a highlighted method/class. From which one is Eclipse fetching and showing the explanations (whether in the pop-up or the javadoc pane, which is a tab beside the console by default).

I mean the method/class descriptions for a whole entity, its parameters, return types, ... are already defined in the source code with the annotations, and the showing descriptions are identical to those annotations' texts. So what would be the use case of adding the doc path to Eclipse's installed JRE settings?