secondary ui what is

Hi,
I was just going through the code and wondering what exactly a secondary 
ui is?

I can think of two scenarios:

1. Primary UI is the ui which get's all the key-presses.
All other UIs are called secondary

2. You have
one primary UI
one secondary UI
one musicbrowser UI
one toolbar UI
one download UI


I guess only scenario one makes really sense.
Please tell me what is right.



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