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now i using smooth-tasks and system tray plasmoids: it is a two groups of applications, and it will be great if can be able to merge them together

This sounds like a feature enhancement for the smooth-tasks plasmoid, which is a 3rd party addon and thus not appropriate for the brainstorm section.

sorry when i had wrote smooth-tasks i thinking abstractly about taskbar

OK- so what will the benefit be if they are merged?

get rid of double showing icons of applications, which use tray and of two places where i must looking for running application

i am not sure but, is aim of tray in showing notifications?

No, the purpose of the notification icon is to show notifications. The purpose of the system tray is to hold "iconified" applications - such as those which you want to run 24/7 like KMail and want easy right-click menu access to common functions, or daemons like KOrganizer or KAlarm which just sit in the background most of the time.

So if you want to merge them, you need to suggest a way to merge them such that no functionality will be lost.