Using segment builder as a trigger in customer automation rules

I’ve been experimenting with customer automation rules for some time, and I’ve encountered Segment building as a trigger option.

I understand that segment builders are defined in backend, and are PHP classes that can create automatically calculated segments based on the customer data, but I haven’t figured out how can I use segment building as a trigger while creating manual segments.

Does anyone have more experience with using segment building as a trigger for creating manual segments?

Thanks.