The rule is that you will be tested on the information presented in the current equivalent of the course . What is at question is your competency in that topic today . The other 72 credits must be allocated roughly equally among the four study areas. One of the four study areas must be a Systems Science-related topic, and one must involve some form of research methodologies. They all must be developed with the advice and consent of your advisor, and, ultimately, your Comprehensives Exam Committee.
Detailed requirements are available in the Catalog.
Tell me about the Comprehensive Exams. As was said earlier, your Comprehensives are four-day set of Written exams, followed by a two hour Oral exam, covering your four areas of study, including any transfer courses . You get two tries. Fail them both and you are out of the program. Each day, for four days, you take a four hour Written exam which covers one of your chosen study topics.