For bonus points: how tall is the ramp at Baja?
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).
mBf refers to the Baltic sea level, which differs from sea level elsewhere in the world for two reasons. Firstly, the Baltic is less saline, and hence less dense, than the North Sea and Atlantic, to which it is connected. Consequently, since it is in quasi-hydrostatic equilibrium with the North Sea, it is also higher than the North Sea. Secondly, in response to the retreat of the Scandinavian ice cap, the land around the Baltic has risen by up to 9 metres per century. The current rate of change being 0.3 to 12.3 mm per year. Put another way, the apparent sea-level has fallen by anything up to 9 metres a century. The mBf is therefore used as the local sea level reference datum for such activities as estuary embankment works, mine drainage, and industrial cooling water intakes. See, for example: http://www.baltex-research.eu/publications/Books%20and%20articles/The%20Changing%20Level%20of%20the%20Baltic%20Sea.pdf