On custom button action we can add link, ancestors but not following segments. I think this could be usefull when the developer wants to add a button to enter to a collection which has the actual record as dependency.
For example, if we have a table of teachers and we want to create a custom button for each record to access a list of students for each teacher, the url would be similar to the following:
teachers/:id/students
Now, it's not possible to generate this kind of custom url