On the destination computer one can install from the USB (or any mounted filesystem) using the --sideload-repo option with flatpak install. It's also possible to configure sideload paths using symlinks; see flatpak(1). Flatpak also includes systemd units to automatically sideload from hot-plugged USB drives, but these may or may not be enabled depending on your Linux distribution.
Each REF argument is a full or partial identifier in the flatpak ref format, which looks like "(app|runtime)/ID/ARCH/BRANCH". All elements except ID are optional and can be left out, including the slashes, so most of the time you need only specify ID. Any part left out will be matched against what is installed, and if there are multiple matches an error message will list the alternatives.
By default this looks for both installed apps and runtimes with the given REF, but you can limit this by using the --app or --runtime option.
All REFs must be in the same installation (user, system, or other). Otherwise it's ambiguous which repository metadata refs to put on the USB drive.
By default flatpak create-usb uses .ostree/repo as the destination directory under MOUNT-PATH but if you specify another location using --destination-repo a symbolic link will be created for you in .ostree/repos.d. This ensures that either way the repository will be found by flatpak (and other consumers of libostree) for install/update operations.
Unless overridden with the --system, --user, or --installation options, this command searches both the system-wide installation and the per-user one for REF and errors out if it exists in more than one.
The following options are understood:
-h, --help
--user
--system
--installation=NAME
--app
--runtime
--destination-repo=DEST
--allow-partial
-v, --verbose
--ostree-verbose
$ flatpak create-usb /run/media/mwleeds/1a9b4cb2-a7ef-4d9b-91a5-6eaf8fdd2bf6/ com.endlessm.wiki_art.en
flatpak(1), flatpak remote-modify(1), flatpak-install(1), flatpak-list(1), ostree-create-usb(1)