I have never seen this?

I see it and I think is the correct mod.

Can someone please explain them?

I'm not an expert but I try.
In SH3D are two category of objects: plan components and furniture.
They all belong to a level.
From plan component only walls (if have enough high) will appear on upper level. The rest only on selected level.
Usually I make multilevel plans especially because is this feature.
Way I use multiple levels?
Sometime at same elevation (for pipes, wires, dimensions, some labels etc.) and I need to see ONLY one at a time.
Sometime at a little high elevation (5~10 cm) for floor to hide some pipes or heating system and again at same elevation but an other level for some furniture.
All levels could be made visible in 3D for a better view.

Unfortunately I'm not a native Java speaker

Me neither!

it's taking me more than a few moments to grok it

I try this for two years and I can't get anything, but I steel insist.

Just curious if I should just patch this for myself

Of course You can try but first You have to learn how to do this.
Then maybe teach me how to do.