The Preview mode allows you to see how dynamic elements in a topic act depending on the Output Tags used. 

To check how the output looks for a certain dynamic element, you need to go to the View tab on the Design View ribbon bar and pick an Output Tag you want to be set as default for this Project 's Preview mode. Then, click the Preview tab at the bottom of your User Interface.

Note that the last Output Tag used will be saved for the current user when viewing all topics in the current project. Other users editing the topic can have different tags set up for their Preview mode.

Two types of Output Tags are used on the screenshots below. The beginner tag was created for the output visible only in the documentation for beginners, while the pro output tag is used to mark blocks that will be visible in the output for pro users.