Flour and DIN standards, what do they mean?

I hope someone can help me out on DIN-standards. I live in the Netherlands and there's an Aldi supermarket close to where I live. As you may know, Aldi is a German chain of supermarkets and most products they sell come from Germany. They sell an excellent German milled patent wheat flour as well. On the package it says 'Type 405'. I know that this number is a so-called DIN number, the German equivalent of ISO-numbers. It's much like in the old days of film photography where a DIN 21 film was the same speed as an American  ASA 100 film. Other than that I can't figure out what the number actually means in relation to other DIN-numbers for flour types. I hope someone can help me out on this.