What is the use of trust depth?
Basically:
In trust setting, all those are mentioned as below-
Depth 0, the list of people who are included in your trust list.
Depth 1, the list of people who are trusted by Depth 0.
Depth 2, the list of people are trusted by Depth 1.
What value I need to set when I am making a custom trust list?
You should probably just let it set on 2. If you set a bigger number, you will end up trusting half of the forum.