What is a valid relationship type between objects A and B when defining a custom report type?

You need to be familiar with what a report type is first. This is a type of template that you can easily follow. It will have certain spaces for the type of objects that you need to place on the report. There are also fields available wherein you can put the needed details easily. Do remember that report types can hold standard objects. Custom objects are also placed.

It would entirely depend on the report that needs to be done. There are moments when the objects or fields that will be placed there will have significant relationships with each other. There are also moments when they do not have relationships with each other at all.