@Steve - that helps some in obvious cases like US Presidents, but it doesn't address M Ross's use on successive owners of a family business described above. This seems very genealogically significant, but I can't think of any project that would manage such a case. In such cases not directly related to an existing project, do people need to get permission to use a succession box? If so, from who? We also don't have geographical projects for every country yet, so who will manage political/royal succession boxes for those countries? I'm inclined to say that #3 should be changed to something more like "Projects may institute formatting and use guidelines for succession boxes that are directly related to their project or commonly used by their project." This would allow more free, innovative use of succession boxes in cases where it's not likely to cause conflict and/or it's not directly related to an existing project.
I also think that the number of people who have asked for clarification on the meaning of "great genealogical and biographical significance"" indicates that clarification is indeed necessary if this is to remain one of the rules. I asked about it partly as a project coordinator who may be one of the people that would be making these decisions for my project in the future. At this point, I don't know what it means well enough to make a fair decision for any cases my project is asked to decide about. And I think that such a vague, subjective standard will inevitably lead to arguments and resentment when projects say someone's proposed use is not significant enough to allow or when different projects use different standards. Clear, transparent guidelines in these cases help prevent this kind of unnecessary conflict. As it stands, I think that #2 may be unnecessary in most, maybe all, cases. Are there specific uses that rule #2 is trying to prohibit? What problem is it trying to solve? As far as I can think of, the main thing that needs to be limited is not how "significant" a succession is, but rather the number of succession boxes on a single profile so that they don't take up too much space, causing readers to scroll really far to get to the actual biography. Dealing with that problem brings us back to my above comments/questions related to possible rule #6.