Pipe multiple tagged messages to program from within mutt

I understand how to pipe the contents of a single message through |. Also I understand how to tag messages in multiple different ways, as described in the help accessible through ?.

What I don't know:

1. How can you pipe the contents of a list of tagged messages to a program?
2. Is there a way to pipe a single message to a single program call?
3. Is there also a way to pipe the concatenated contents of all messages to a single program call?

From the documentation:

pipe-message (default: |)

Asks for an external Unix command and pipes the current or tagged message (s) to it. The variables $pipe_decode, $pipe_split, $pipe_sep and $wait_key control the exact behavior of this function.

So, depending on the 3 settings ($pipe_decode, $pipe_split and $pipe_sep), you can accomplish your use cases.