dub.selections.json & optional dependencies: How's it work?

[jmh530]

I'm looking at the dub package format [1] about optional 
dependencies and it says:

"With this set to true, the dependency will only be used if 
explicitly selected in dub.selections.json. If omitted, this 
attribute defaults to false."

And it occurs to me that I don't know anything about how 
dub.selections.json works.

I would think dub optional dependencies work such that if there 
are no functions being called/compiled that import an optional 
dependency, then the dependency wouldn't be included. How is it 
different from `dmd -i`?

The dub.selections.json in some of my projects look like a 
listing of the dependencies and their versions. Above should 
imply that the optional dependency would only get included there 
if the import is actually used somewhere in the project. Is that 
correct?

[1] https://dub.pm/package-format-json.html