Trust signature domain

[John Lane]

I'm trying to experiment with trust signatures but I can't work out how
the 'domain' question is used ?

I think I understand what it is for, but I can't enter a value and get
it to work.

I have a key A that has signed B at example.com and C at example.org

If I tsign A at level 2 with the domain blank then B and C are fully valid.

If I tsign A at level 2 with a domain of example.com then neither are
valid. I expected B to be valid.

>From what I've read, I think this value might be a regular expression
and need to be entered in a certain way.

Any pointers appreciated.....