POST parameters v-wn and v-uiId

I know that v-wn is a window name (tab name) and is used by Vaadin to identify from which browser tab the POST is sent. I though that v-wn is attached to every request. But I have button in my application. When I first time clicked the button, a lot of v-* params was send. But when I clicked second time, only v-uiId is send.
What is meaning of v-uild POST parameter?

Hi,

the v-uiId parameter contains the ID of the UI (running numbering of UIs within the same session), so if you open a second UI, it should have uiId 1 and so on. It’s used for at least mapping the requests to the correct UI instance in the session on the server-side, but there might be some other uses for it too.

The window name is not enough to uniquely identify a UI instance, because a single HTML page may contain multiple embedded Vaadin UIs. Hence dedicated IDs.

Thanks. Your answer realized me that I had wrong idea that browser tab=UI.