You may assign a PIN to either a playlist or a song. Entering that PIN will begin playing that "thing".
So if you assign a PIN to a playlist, entering that PIN will being playing that playlist and all its sub-playlists. If you assign a PIN to a song, entering that PIN will begin playing that song.
I take it you don't have an Empeg yet? Most owners say that once they've got the Empeg, they find that they don't need the PINs at all. It's fairly easy to navigate to a specific playlist, artist, or album, assuming your playlists aren't too complicated.
For instance, your level of "solo" and "group" in the playlists might not be necessary, and would be just an extra step when navigating the tree. If you think it's important, by all means leave it in, but in my experience, the fewer levels to the tree, the better.
You'll find that most of the time, you just listen to the whole thing shuffled, or you listen to a specific "mood" playlist, or you alphanumerically-search for individual songs. Trying to remember a huge set of PINs is kind of a waste of time. PINs are useful for tagging some specific favorites, but that's about it.
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Tony Fabris