These articles contain good information about this metric:
Hutchins, B. I., Yuan, X., Anderson, J. M., & Santangelo, G. M. (2016). Relative Citation Ratio (RCR): A new metric that uses citation rates to measure influence at the article level. PLoS biology, 14(9), e1002541.
Bornmann, L., & Haunschild, R. (2017). Relative Citation Ratio (RCR): an empirical attempt to study a new field‐normalized bibliometric indicator. Journal of the Association for Information Science and Technology, 68(4), 1064-1067.