A German ligature that originally stood for 'sz', but can be transliterated as 'ss'. It is never found in Swiss German. It looks like a B, but sounds like an S. Compare to the boring language known as English, which only has the ligatures Ã and Å, which are rarely used.
The only other flour used in traditional pumpernickel is wheat flour, which is used as an ingredient in the sourdough starter, a pre-fermented flour, water and yeast mixture that combines with the rye flour to hold the bread together.
Cake flour, bread flour, whole-wheat flour, pastry flour, self-rising flour, and instant flour are all different things and shouldn't be used interchangeably with all-purpose flour. And we're not even getting started on all the flours that use grains other than wheat.