Asset files include, but are not limited to CSS, JS, and Image files. These files can be found and managed under the Assets section in Themes/Files.
The templates section contains your sites Page and Layout templates.
Page templates are your main HTML template files. Page templates are used when you want to provide the same layout and style for a page, but the content is likely to differ between them. To enable Governors content management features, your page will need to include GovTags.
For more information on how GovTags work, see the Theme Documentation.
Layout templates are the HTML files that are included on multiple pages. Great candidates for Layout files include headers, footers, and social media icons; Sections that are included on multiple pages of the site, but their implementation and content are unlikely to change.
The Modules section contains module specific page and layout templates.