How do list threads work for MIME digest?

[John Cox]

Excuse the Newbie question but I am a newbie:

Are the threads held together based on the text of the subject line or
is there some magic in the header  that makes it work?
The reason I ask is that I get  the MIME digest and if all that is
required to maintain a thread is the subject line--no problem but If
there is some kind of non-subject line tracking involved,  how do I
reply from a digest.?

I'm using TB .4 and it forces a reply to the entire digest and won't
allow a reply to a single message.

John