The man page for new-template says:

The first TYPE parameter is the name of an AppStream component type. For a complete list check out the documentation[2] or the help output of appstreamcli for this subcommand.

But appstreamcli new-template --help says nothing about types, and the referenced HTML documentation page doesn't contain a list either: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html