Thanks for this clarification, Malcolm.
I deleted several quizzes (assessments) from an originating course and was surprised to learn that my gradebook categories had disappeared from the receiving course when I performed a restore. The new (receiving) course had a gradebook with no categories at all!
BTW, I deleted the quizzes in the old (originating) course because I wanted
to clean up the question bank and get rid of old quiz questions that I
no longer use.
As I view it, here's the problem/dilemma: I can drag these old quiz questions from course to course - or I can delete them but I will have to recreate the gradebook for each new course.
At present, I rename all unwanted/deletable quiz questions with the preface "Old" but I don't in fact delete them.