Create my own Callout type, for example, "Summary" callout?

Yes—if you mean a Markdown callout, you can create a “Summary” callout by using the standard Markdown callout syntax:

> This is a summary of the player’s career.

However, TheBrain’s documentation specifically confirms the standard callout form > [!NOTE], but it does not confirm which custom callout names are supported or whether SUMMARY receives special styling.

So:

• > [!NOTE] is documented and supported.
• > [!SUMMARY] may remain a plain blockquote or may render according to the editor’s supported callout types.
• The documentation does not confirm a user interface for defining custom callout types or custom callout styles.

You can still use a clearly labeled blockquote safely:

> **Summary**
>
> Brad Lidge was a right-handed relief pitcher...

That will provide a consistent visual summary even if custom callout styling is unavailable. TheBrain stores standard Markdown callouts in the portable form used by GitHub and other Markdown editors.

“Portable form” means the formatting is stored using widely recognized Markdown syntax, rather than a format that only TheBrain understands.

For example, this callout:

is recognized by several Markdown tools, including GitHub and many note-taking applications. If you export or edit the note outside TheBrain, the underlying text remains understandable and the callout formatting has a good chance of being preserved.

In practical terms, standard Markdown makes it easier to:

• Export notes.
• Edit them in another Markdown editor.
• Store them in Git or another version-control system.
• Import them into another compatible application.
• Keep the content readable if you ever stop using TheBrain.