CustomHeader control allows you to append custom content to <head> section of your TITAN application.
Although you can place whatever content there, it's highly recommended to use this feature only for web development purposes i.e. including additional styles and scripts.
Once you inserted your custom content, it is persisted into your database and applied on every page of your application.
To update custom header, go to Admin Panel → Website → Settings → Header & Footer.
CustomHeader.ascx can be replaced during update process. That's why you should not attach your additional styles/scripts and other template files directly in the file contents. Instead, use the Admin Panel UI as image shows.