RDs are Referring Domains, you could have 1 Referring Domain and 10,000 backlinks from a sitewide footer link on that one domain for example. I always work on the basis that if you got a link from herpderp.com, then having a second link from herpderp.com is unlikely to add as much value as the first link did (unless it's placed in a better page etc etc). It's better to get 1 link on multiple domains as oppose to multiple links on fewer domains. So focus on building links on new domains everytime, this will increase both RD and Backlinks