If your report contains a metric or a dimension that has more than one source connected to draw values from, you can now select which of these data you want to use in the report.
For example, you want to build a report featuring Ad cost. You have "Website user behavior data" and "Cost data from Google Analytics" sources connected. Both sources contain ad cost data.
Now, if you want to use only one of these sources for Ad cost in your report, you can quickly choose which one it would be: just click the checkboxes in the new contextual menu right in the report builder: