Water levels: What does "mBf" mean?
It is "mean sea level above the Baltic Sea", in meters (source on page 109). 

That explains why it is used mainly in East-European countries.

Martin Ekman - The Changing Level of the Baltic Sea during 300 Years - A clue to understanding the Earth shortly mentions some historical 'zero points' (page 111). 

As mentioned in the comments: 
mBf means méter Balti felett (Hungarian for meters above Baltic Sea). They also use meters above the Adriatic as a datum (https://en.wikipedia.org/wiki/Metres_above_the_Adriatic, mAf from méter Adria felett).