If a student is getting a major in one of the three broad areas, and a lot of students are, then getting a minor in Computational Science will "round out" their education in the other two areas. So, for example, a student who is majoring in Geography, and then enters the IGMCS program, would take courses in Math/Statistics and Computer/Information Science to get their minor in Computational Science.   Students in one of the other two ("non-domain") areas would round out their training in an analogous way.