hey Rolo-
The APPLICATION KEY just opens the menu of what you've recently clicked (at least inside Sonar 8.5.3, as far as I know- don't know X1)- if you've clicked on the tracks bar, when you press the application key it opens a track menu, just hitting a single letter like S or R accomplishes what that key is short for in the menu. So if you swipe a set of tracks to solo, Application key> S will unsolo all those tracks. As far as what Chappel says, yeah, the toolbar button does that too, in many ways much easier, but it requires mousing- which is what this here tip tries to AVOID! I really like shortcuts... better than mousing!
A while ago there was a thread about finding a keyboard shortcut to unsolo all- and the only way we thought it could be done is cntrl A> shift S (select all, solo all) then shift S again (unsolo's all). This only worked for me because all my track shortcuts are mapped with "shift" as a modifier. But there wasn't a KEYBOARD shortcut available to "safe all" or unsolo all" in a simple form, (like clicking the toolbar- but just a key).
I'm still learning how to use the Application key in Sonar- the menu it gives you, and the hot keys which that menu offers- depend on what you've recently clicked on... so it's not as perfect as just clicking on the toolbar. But it's one more way to avoid the mouse(!) which is good for me! Since I usually have a cello, electric guitar, etc, in one hand, a bow in the other, a beer in my lap and a fattie in my mouth- plus a remote control or two...
I'm always looking for less work to do- you know... I'm a musician.