Is it possible to create a relationship between multiple fields with Meta Box?
Let's say I have a list of Courses, a list of Lessons, and a list of Topics. I want that when I select a Course, the Lessons list only shows the lessons from that course. And similarly, when I select a lesson, the topic list only shows the topics of the selected lesson.
It would be like three cascading lists, where the content displayed in the 3rd list depends on what you have selected in the preceding lists.
It is not possible to create relationships between custom fields. The extension MB Relationships helps you to create connections between objects (post, user, term), not the fields. Please read more on the documentation https://docs.metabox.io/extensions/mb-relationships/