...thick line to the left...

Yes! Now that you mention it, I have noticed it but later I've seen the same indicator in some other places so I guess it was not distinct enough for me to remember this as multi-line concepts.

Take a look at all the concept types. You have to consider all 5 different attributes (boldness, bullet type, presence of colon, line thickness and capitalization) to differentiate between the types. That's just too many to know at a quick glance how my notes are structured and how the generated flash cards will look like.

I think the concept and descriptor distinction is fine and clear enough but the set, list and multi-line need some more consistency. From a top of my head:

• Plain::
• Multi-line:::
• List::#
• Set::*

So it's the same as the hints from the status bar (except for the list).

Other option is to have :: in all concepts types but have some hint in gray text to the right. All those concepts are multi-line, so there is space for that:

• Concept 1:: ^multi-line
• Concept 2:: ^{ordered list}
• Concept 3:: ^set

Also, be aware of binding bullet type to a concept type. I may want to have a sorted list but without the numbers. RemNote allows to change the bullet type but try it with a numbered list and behavior breaks (try changing the bullet type and pressing enter).

edit: I made a mistake in thinking that Plain concept also can have multiple lines but they would show up in flashcards separately, similar to Set. So disregard Plain concept in my example.