EITHER
Press the Download Comparison Report button at the bottom of the modal. The report includes current members (concept names and IDs and total number of members) of both reference sets, members in common, and distinct members of each reference set.
OR
Press the Download Change Report button. The report includes members added or removed during the session (available to author only, while reference set is "In Edit"). See - Launch Comparison for Editing - author.
Report(s) will be automatically downloaded to user's computer.