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.
On bags in supermarkets you will mostly see patentbloem and tarwebloem, the first is usually used for pastry and cookies, the second also for bread baking (see list above). If you want good quality and the right type of flour for your bakes it is best to visit a flour mill.