> Does "levels" mean "depth"? To give the feel how deep in the subject you are?

Yes! Level means depth.

> Don't stacked "parents" give that feel?

No! The stack doesn't give me that feel, because visually, it doesn't look like a stack. My brain perceives a series of plain text lines as a block of information. There's a reason we add symbols (numbers, discs) to checklists. We want them differentiated from normal text blocks. Of course you don't necessarily need to use symbols, you could also make use of relative positioning (between levels) or color (varying the intensity of the same color as you move up and down the levels, or use a color theme), etc.

Regards!

Got it. Thank you very much!

You might want to checkout the way 'Thebrain' 1) can show related thoughts using layers of transparency. Our brain reads these indirectly and helps providing contexts.

1) https://www.thebrain.com/

* not affiliated in any way

Oh! The Brain. Remember it since like end of 90s I guess) Glad they are still in the game.

Will check closer that transparency feature you talk about.

Thank you!

It doesn't show if a card is stacked or not.

Got it, yes, I see now. Will think about it. Thank you!