In a conflict of interest where I represented a person previously, what does “substantially related” mean in terms of disqualification?

The term substantially related is found in Rule of Professional Conduct 1.9, titled, “Duties to Former Clients.” The conflict of interest rules are set forth in Rules 1.7, 1.8, and 1.9 of the Rules of Professional Conduct. Rule 1.7 is the general conflict of interest rule involving concurrent conflict of interest. Rule 1.8 is special conflicts of interest rule. Rule 1.9 is the conflicts of interest rule involving former clients.