I need to create a relationship between a custom field that is part of an administration page and a user, as a custom field called "Classes" was created and now I need to relate it to the user, as a class can belong to several Users.
It isn't possible to set a relationship between a custom field and a user. You can set a relation between objects only. They are post, user, term.
Please follow the documentation https://docs.metabox.io/extensions/mb-relationships/
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