Nested citations allow a writer to group citations on a specific topic. For example, if your paragraph mentions concepts from three articles on cholesterol testing chemicals, you could use an endnote at the end of that section in the text. Then at that end note in the reference list, include all three articles. Fore example:
1.(a) Jones, J.; Jones, B.; Smith, S. Transfusion 2011, 51(4),1120-1133. (b) Smith, P. Clinical Chemistry New York, NY. Pearson 2014. (c) Anderson, T.A.; Reeves, K. Che. Matrix Bul. 1999, 20(6), 9-14.