batocera has 2 partitions.
the first partition stores the system.
the 2nd partion is for the data.
when you install batocera, there is only the system partition.
batocera creates the 2nd partition for the data with the remaining space.
in case batocera cannot create or mount a such partition, it mounts a tmpfs instead of this partition. in that case, of course all data are lost when you shutdown batocera.