This system is used to classify colleges and universities into some thirty plus categories based on student population, degree type, and broad area of study. There is a PDF flowchart that explains the classifications: [PDF] http://carnegieclassifications.iu.edu/downloads/CCIHE2015-Fl...

I came across this on The Research and Education Networking Information Sharing and Analysis Center (REN-ISAC)'s site as a method of calculating membership fees as a function of institution size. https://www.ren-isac.net/membership/fees.html