Here you can insert additional scripts like the Google tracking code, CSS code, JS codes, and HTML tags. for example.
Any code you place here will be appeared just after the opening body tag.
It will be added before the closing body tag in the theme.
You can use custom CSS to add your styles (code tweaks) or overwrite default CSS from a template or extension. This option is useful for small design changes.
There are several ways of adding your own custom CSS or custom JS to a Helix-based theme. For further information, take a look at the Customization section tutorial.