From the ComScore press release:
‘Visits’ an Effective Gauge of Frequency
While each of the “visits” metrics offers a different measure of frequency, the “average visits per visitor” is the most illustrative of return visits per unique individual during the course of a month. Used in concert with the “unique visitors” metric, this measure can help give a more comprehensive view of a site’s performance.
Or to state it another way, keeping track of how frequently someone visits a particular Web site is the most effective way to measure how frequently that person visits a particular Web site.
Like, for example, the way that every modern Web analytics package has tracked visits and visit frequency per user for a number of years now.