Syncing playlists on mobile with computer does what?

[beric0]

So what exactly happens when you sync a playlist with your computer? If there are no local files in the playlist, will it affect anything on your phone? Also, if I do have local files on the computer, but identical files on the phone, when the playlists sync, what will happen? Does it still download another copy of the file from my computer to my phone just for spotify? I think its kind of annoying that I can't play a local song on my phone if i add it to a playlist from my computer or vice versa for identical files that I have on both the phone and computer.