The "Custom Selector" option allows you to highlight a specific section, outside of the comment section, e.g. in the post/page content.
Let's consider an example.
1. We filled in the .test class in the "Custom Selector" field
2. Somewhere on the page/page content we've added the code with the .test class
3. As a result, it'll be automatically highlighted:
Important: the class can be applied only on the <code> and <pre> tags.