What is this icon?

[Reaqer]

Ok after much searching I can't seem to find the answer to this.

When you go to a track and you swipe it, it shows a variety of options you can do. The first one is where you star the track, but the second one shows a few rectangles on top of each other with a plus sign on the top. I pressed it and it moved the track to the top right corner of the iPhone screen, but nothing happened that I could notice.

What is this icon and what does it do?

[Sweetmo]

The icon allows for a list of songs to be played as they are selected.   While listening to a genre or playlist it will insert the songs.  When it runs out of inserted selections the playlist the random selections pick up again