The Ad Price Assignment in the second tab allows you to assign Contents with Ad Prices.
The assignment tab offers a Content section and an Ad Price section. In both of them you can Search Contents and Ad Prices / Ad Specs.
To create assignments you can simply hit the + buttons for every relevant record and then click Save Assignments in the lower right corner. The assignments will be created for every possible combination given by your selections, e.g. in the screenshot below 4 assignments would be created.
After successfully saving the new assignments a toast will confirm that to you.