Look at this - a screenshot of the sidebar in iTunes 12.7.3.46 on High Sierra:
There are two smart playlists and 3 standard playlists.
The symbol for 'Playlist' is familiar. But what do the other two symbols mean - the one with 4 lines ('now') and the other with 4 lines and a downward arrow ('Purchased')?
Mac mini, macOS High Sierra (10.13), iTunes 12.7.3.46
What you should notice is that the 'Purchased' playlist is above the smart playlists, even though standard playlists are usually placed below smart playlists. So what explains the special status of that 'Purchased' playlist? I am almost sure I found the answer:
In iTunes 11, purchases were shown in the sidebar, like this:
In iTunes 12, purchases have been moved to Account > Purchased.
After moving my iTunes 11 library to another computer with iTunes 12, the "old" 'Purchased' playlist has been preserved and is made to stand out with its own type of icon:
Regarding the other playlists with the 4 lines, it is still not clear what makes them different from the standard playlists...
Chances are that now has a mix of content that includes things other than Music.
tt2
I checked the total of nine playlists with the 4-lines-icon and – yes, you are right, they all contain a mix of media kinds.
😎
tt2
Three types of standard playlist icons (symbols)
