[cross-project-issues-dev] Technical question on "optional features"

[David M Williams <david_williams@xxxxxxxxxx>]

I'm wondering ... 

While testing the Europa Site, when
I select a whole bunch of stuff, I eventually get a dialog that says "The
following features are optional, do you want to install them or not"
(to paraphrase). 

How does this happen? I don't recall
ever seeing this optional part of the sequence before. 

I'm asking because I want to do it too
:) 

I'm just wondering if it is a side effect
of  all the plugins in one of these features happen to be optional
to all other plugins, 

or, if there is some way in features
or site.xml to specify an "optional" feature? 

Thanks in advance.