I have added plenty of CD singles, and related artwork. I agree with @jesus2099. This is just another example of MB messing with language a bit. What they mean is what we expect - thin MUSIC CD single cases. The thin things like in the example photo. I expect this is why they show the different types behind the link.
They are both physically “slim” in size.
What is really missing is a way of properlly attaching a type to the artwork. The closest we can do with that image is a “front \ spine \ back”. Which seems odd as there is not really a “back” to a slim CD case. Artwork types are way too limited. (see also missing gatefold artwork type that could be used for a digipak…)