Rather OT - what is this icon?

[Timothy Murphy]

What is the syringe-like icon which appears on all plasma pages
in the "title bar" at the top of the page, second place on the left?
And what exactly is its function?
When I left click on it, it turns into a small white disk
with a dot in the centre.
If I click on it again, it turns back to the syringe icon.
When I right click on it, nothing happens.

More generally, is there a page online somewhere
that outlines the anatomy of a plasma page?

When I hover over any icon in the panel I am told what it is;
and if I right-click on it I'm told more about it.
But when I hover over, or right click on, icons in the title bar
I get no response.
That seems unfortunate to me.