One of the goals of the migration is to move the files used by your current on-premises installation to Author-it Cloud. The new location for your external files will be Files, which you access from the Author-it Cloud Portal.
Files is the location for your publishing templates, image files for linked graphics, and other external files that you need to move to Author-it Cloud.
When publishing to HTML (HTML Pages or XHTML) or Web Help, Author-it checks for the Plain HTML folder in the Templates folder. You need this folder in your Templates folder in Files even if you do not publish to these outputs. When you are mapping your files, you will see the following message if the Plain HTML folder is missing:
For more information see Publishing requirement.