The R80 platform is built based on revisions. On the back-end, every time an admin publishes his changes, a database revision is created with only the delta changes that were made by the admin. This is an important building block of the platform that we utilize in many ways. You can read more about this at R80.10 Architecture Overview
So when you click on Install Policy and select the policy to install, SmartConsole is able to show you all the changes that were published since the previous policy installation.
Clicking the link brings the revisions view, filtered to only show the relevant sessions that happened after the revision that was installed on the gateway. Inside the revisions view, the bottom pane pulls the changes that were linked to the selected session from the audit logs.
You can open this dialog per specific-gateway as well.
This gives you a way to verify what exactly will be pushed to each gateway, prior to installing the policy. You can review these changes, and potentially decide to postpone the policy push in favor of changing some of the elements.