[itsme]

.MSS is an XML file without song files, see attachment above
it is created by selecting a setlist and calling "Share - Export song list" (Teilen - Songliste exportieren)

.MSF contains the list of songs including the song files 
it is created by selecting a setlist and calling "Share - Export songs and files" (Teilen - Songs und Dateien exportieren)
(what BRX proposes as 'setlist as MSS with the song files')

When importing these files, they are applied to the database they are imported into, the 'destination database'. Songs without SongID are identified by the combination of title and (first) file. Songs with SongID are identified by SongID. In both cases the destination database is searched and the songs in the destination database are used. This is what Mike is warning of: if you import an MSF file into a different database and a different song is found, the file from the MSF file overwrites the one in the database. This has the potential to completely confuse the destination database if you don't know what you are doing.

In case you import an MSS file and no matching song is found, MobileSheets creates an empty song, gives it the titel from the MSS file, adds a single blank page and adds that empty song to the setlist.
Open question: how to find those empty songs in a huge database? 
If I create an empty song manually, I have to add at least a blank page. 'Blank Page' is written into the file path, so I can find it by selecting 'File Paths' as search field and search for 'Blank'. For empty songs that were created by importing an MSS file this doesn't work.