When researching backlinks in Backlink Explorer, you're provided with two different quality metrics from Majestic for determining each link's usefulness to your link-building efforts: Citation Flow and ACRank.
What is Citation Flow?
Citation Flow is a rating of 0 to 100 that appears alongside backlinks researched using the Fresh Index. The higher the score, the more influential the link is, based on how many other websites link to that URL. Not all links are weighted equally, with Majestic's algorithms determining "strong" and "weak" links.
This metric can also be found in Research Central and Link Manager.
What is ACRank?
ACRank ("A Citation Rank") is the older version of Citation Flow, also pioneered by Majestic. This metric only appears alongside links in the Historic Index, which doesn't have access to the newer data from Citation Flow. This also determines the importance of a URL, but uses a simpler algorithm to determine that number.
Unlike Citation Flow, ACRank runs on a scale of 0 to 15, with 15 representing the most influential links, and does not weight URLs as "strong" or "weak." As noted, this metric only appears in Historic Index searches through Backlink Explorer and has otherwise been replaced by Citation Flow elsewhere in Raven.