flatpak can operate in system-wide or per-user mode. The system-wide data is located in $prefix/var/lib/flatpak/, and the per-user data is in $HOME/.local/share/flatpak/.
In addition to the default installation locations, more system-wide installations can be defined via configuration files /etc/flatpak/installations.d/, which must have the .conf extension and follow the format described below.
The installation config file format is using the same .ini file format that is used for systemd unit files or application .desktop files.
All the configuration for the the installation location with name NAME is contained in the [Installation "NAME"] group.
The following keys are recognized:
Path (string)
DisplayName (string)
Priority (integer)
StorageType (string)
[Installation "extra"] Path=/location/of/sdcard DisplayName=Extra Installation StorageType=sdcard