This facilitates apps being able to transfer files back and forth using drag-and-drop or copy-paste without having to give them blanket permission to read all files. https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.FileTransfer.html
As far as I can tell this is not something that needs to be implemented in the portal backend. xdg-desktop-portal provides that without needing a backend implementation
Oh OK cool. No wonder I couldn't find any evidence of it in our portal. :)