According to the announcement:
“When you upload your customer list you’ll get a match rate—this is the percentage of your customer list that is usable with Customer Match. Previously, you had to wait to see what your match rate would be. To help you identify and troubleshoot issues faster, you can now see your match rate instantly. In addition, we’ll show you the match rates of previous uploads so you can see your historical trends.”
Google recommends that you add as much customer information as you can in order to get the most out of your customer lists, ultimately increasing your match rate.
“Advertisers who uploaded two types of customer information saw an average list size increase of 28%, and with three types they saw an increase of 35%.”
For more information on Instant Match Rates, you can read more here.