Can anyone explain the "Notification Area" configuration interface

[Chris Green]

I have been esperimenting a bit with the "Notification Area" panel
item in xubuntu 16.04 and I'm finding the configuration very
confusing. 

When I open up the configuration for "Notification Area" I'm presented
with a table of "Known Applications" each of which has a tick box for
"Hidden".

The problem is that this list doesn't seem to reflect reality in any
way that I can understand.  There are lots of applications listed
under Known Applications that don't actually appear in the area on the
panel.

... and what is the point of the Clear Know Applications button, why
would I want to do that?  Do they re-appear when an application
actually runs or what?

It's all rather mysterious!

Finally, where is the configuration stored?  E.g. where is the list of
Known Applications kept?

-- 
Chris Green