When you add content to a report, either in one or multiple Sections, we'll total up all the numbers for all the clips in the Metrics Summary page.
If you'd like to see just the metrics for a single Section of coverage, you can create a new report and quickly add those clips to see just the metrics for the selected items.
First, create a new report in your account (we'll call this the metrics summary book) by clicking on the "create new report" button from the main Reports page, or when you're inside a folder.
Open your original report and the section of coverage you'd like metrics for and copy them over to your blank report.
You'll now have a report with a metrics summary just from the clips you copied over. You can even take a screen grab of the metrics summary page and insert that as a custom slide to your other book.