Exporting a store copies the internal store database, 'compresses' it slightly, then renames it appropriately. The files are in the binary format that Isar uses, so they cannot easily be read or modified.
They can have any file extension applied to them - '.fmtc' is used in the example application. The name of the file dictates the name of the store that will be used when importing (without the 'export_' prefix, if still present).