VirtualDub; "stream" input for filters; how-when-why?

[nji]

Recently
https://forum.doom9.org/showthread.php?p=1893728#post1893728
I discovered a door I didn't know it exists:

Up to now I thought the filter chain was ... well ... a chain:
For a filter:
Implicit input: the output from the previous filter.
Additional parameters by filter's GUI.
Output: 1 implicit one.

Now I discovered the right mouse button on filters...
Forking and re-combining different "work ropes".
I searched for information about when and how to use that
but found nothing helpful.
(Maybe the wrong search terms, as I don't know them)

Maybe someone can point me to a source of information?

To take it systematically:

The "forking" of the "content stream" makes only sense
if the (manipulated) parts are resembled in some way later.

I know only of two filters that take more than 1 input "stream":
merge layers and blend layers.
Are there any other?

Generating the "ropes"/ "streams" only by naming an output
and by fflayer?

(All this strongly reminds my at Douglas Adams' Hitchhiker's Guide,
where the info about the Earth's pending destruction were ... you know where ;-))

Where is the info, where are some typical examles, please?

[shekh]

Well my opinion this is one of unfinished/abandoned features. Maybe Avery Lee was just prototyping something to keep on par with Adobe? :)
There is definitely a lot of effort put in filters pipeline to support this mechanism but I doubt there is any 3rd party filter or anything.

[nji]

Well my opinion this is one of unfinished/abandoned features. Maybe Avery Lee was just prototyping something to keep on par with Adobe? :)
There is definitely a lot of effort put in filters pipeline to support this mechanism but I doubt there is any 3rd party filter or anything.

So no information anywhere?
(Perhaps have to look on Adobe filter pipelines...)

Still I used that mechanism for myself to solve a problem with VD
that I found no other way to solve:
https://forum.doom9.org/showthread.php?p=1893865#post1893865

And in another referred thread it is used to excess:
https://forum.doom9.org/showthread.php?p=1886614#post1886614