What is "collection songs" exactly?

[okaniso]

I have all my songs locally as ogg vorbis files. But when I try to add a cover for a song I often get
"Album cover editing is only available for collection songs."
So my question is, exactly what are the conditions for a song to be a collection song?
(I've tried to find a pattern when it works and when it doesn't, but I failed.)

I'm using strawberry version 1.0.14 (from Debian bookworm)