Here's an explanation of the different columns in the "Additional Account Details" page in "Partner Management" of the Salesforce DMP Console.
Segment Population is the device population of this segment from the Manage Segments screen. This is a count of all devices that are part of the segment based on the segment's rules.
Matchable population is the Segment Population minus those devices that cannot be matched to a partner. This includes Safari cookies, as per default Safari deleted all cookies after each page impression. A DSP Safari cookie will never show up again and thus can't be served ads to. Many DSPs also don't accept mobile identifiers (IDFA and AdID). So in those cases we also deduct them.
Segment Match Rate is the percentage of users from the matchable population that we have matched the DSPs cookie against within the last 30 days. We only send these users as older users are not accepted by the DSPs.
More info on user matching can be found here:
Cumulative population is the population we expect to see in DSP after all the previous considerations. In reality this can differ due to circumstances outside our control. E.g. Google only counts users that have been active in Google's universe on at least two different days for its population/forecasting. For full refresh connections the cumulative population is the population we send every day.
Added and Deleted are those users in an incremental integration that we have sent on a specific day to be added to or deleted from the segment.