Change Reports allow you to capture the differences between crawl runs, such as ratings, prices, or product offerings. A Change Report will run every time the extractor runs.
To create a Change Report, you'll first want to go to the Reports tab and then click the New Extractor. This reveal the Create a new Report screen, where you can set Report Type to Comparison and then select the primary extractor to compare against as well as name your report.
After clicking Apply and continue, you'll want to continue to the Report Builder page. Here you can select extractors and columns to add from the dropdown, including metadata such as Extraction Date.
To set which field to match on, you click the Primary Key icon next to the column header, which will turn gold. To set the Primary Extractor to compare your other extractors against click the star icon next to that extractor's name, which will turn gold as well. While editing, these columns headers can be renamed also are rearrangeable with drag-n-drop.
Once you've setup your columns, you can click Save and Run to get your Comparison Report.