Here is what they say they consider “Similar to ARWU, institutions are ranked according to their academic or research performance in each subject field. Ranking indicators include alumni and staff winning Nobel Prizes, Fields Medals and Turing Awards, Highly Cited Researchers, papers indexed in Science Citation Index-Expanded (SCIE) and Social Science Citation Index (SSCI). A new indicator, the percentage of papers published in the top 20% journals of each field, was introduced.”
This rating scale appears to reflect “impact” of each particular department on their field. It may be a better indicator of suitability for graduate training then undergrad but it does tell you something about how “good” a particular department is in terms of research and training in research.