assets
The assets folder contains public resources. If you are familiar with the directory structure of extensions, this would be the Resources/Public folder. This is the place where you can put your CSS, JavaScript or image files inside. In order to include these in your template, you must use custom Content Block ViewHelpers.
This folder will be symlinked or copied to Resources/Public/ContentBlocks/<vendor>/<name> of the host extension. This is to keep the modular style of a Content Type but still leverage the TYPO3 asset publishing mechanism.
When using a version control system, that path should be ignored by adding this
line to the host extension's .gitignore file:
/Resources/Public/ContentBlocksicon.svg
This is the icon for the Content Type. There is a fallback to a default icon, but it is recommended to replace it with your own, custom icon. You can find many official TYPO3 icons here. Allowed file extensions are svg, png and gif (in preferred order).
icon-root.svg
This is a special icon for Page Types for the "is_siteroot" state. The same logic applies as for the standard icon.svg.