Can somebody help explain the meaning of each channel in view?

When we created new view to filter specific tickets like the ones which are not triggered into any group (We call them "No Group" tickets), I got confused on the channel description.

For example, one ticket was "The follow up" of previous ticket, and I finally found it's in "Closed ticket". What does it mean?

I attach all channels information as below. Please somebody from Zendesk can help explain them.

I know Rob has written before

https://support.zendesk.com/hc/en-us/articles/4408836378394-Understanding-ticket-channels-in-Explore

But it can only help solve part of my problem.

Thanks