Option "system block" missing in display settings for pane and variant

Issue Summary

I would like to set a pane to use the "system block" setting because I need to have a block loaded by an ajax call through the Ajax Blocks module. It is suggested to use that setting and after a lot of searching I finally found the place where it should be set (Content -> Display settings -> Style -> Change). But there is no such option as "system block". All I see is "No style", "Custom style", "List", and "Rounded corners" in the variant style and additionally "Use display default style" in the pane's style.

Am I missing something? Is there another module I need to activate?