What is this icon next to the compilations folder?

This is on my Denon Heos App streaming from my Plex Media Server

Various Artists? Three’s Company? Malcolm in the Middle?

Hard to say, but I’m going with Various Artists.

lol. Hilarious. Yah, I get that, but why is it picking up that it is a Compilations folder? Is it that the folder is called Compilations?
Is it that the tracks inside are tagged as part of Compilations? I have a different folder called Various Artists that doesn’t have the same icon?
Is there any documentation by Plex anywhere about this feature?

This is not a Plex feature.
If it is anything, then it’s a feature of this Denon app.

I assume the Denon app simply uses the DLNA part of Plex.
And with DLNA, every app manufacturer does what he likes. It is hopelessly underspecified.