This is a Syntax definition for Drafts. It recognizes a small subset of the BBCode markup language, namely:
[b]bold[/b][i]italic[/i][u]underline[/u][s]strikethrough[/s]- URL (link)
[url]https://example.com/[/url] - URL (text)
[url=https://example.com/]Example Text[/url] - Image
[img]http://example.com/someimage.jpg[/img]
This particular subset is useful when composing emails and event descriptions for Scoutbook and contains all of the markup they specifically support.
Download the Mini BBCode Drafts Syntax File and open it with Drafts. For more information about managing Syntaxes, check out the documentation for Drafts.