You can find useful informarion about that in the Github upstream URL https://github.com/ximion/appstream-generator/blob/master/docs.

On Apr 15, 2018, at 23:19, Christopher Davis via arch-devops <arch-devops@lists.archlinux.org> wrote:

Hello arch-devops,

I am curious about the files used by archlinux-appstream-data. My goal
is to create a parabola-appstream-data for Parabola GNU/Linux-libre.
According to some people in #parabola on Freenode, the files the
PKGBUILD for archlinux-appstream-data are generated on your servers with
appstream-generator. Could you provide some information on your setup
for this?

Regards,

Chris