I gleaned these strings from AppleScripting. My favorite is "MPEG audio stream" for a Radio Stream because I can keep streams from being selected for a smart Playlist. They tend to just keep playing and I don't edit them out -- I was just about to write a 'script to do that for me automatically, when I stumbled on to this hint.
PS: If you are not sure of the "kind" of a track, copy and paste this to Script Editor, and run it while your "mystery" track is playing. It will display a dialog box with the kind of the playing track. You can copy and paste the info from the dialog:
tell application "iTunes"
display dialog "Current track's kind is:" default answer ((get kind of current track) as string)
Get info on the track and look for the value of Kind: in the Summary panel.