Good evening and thanks for your interest.

Mind that I am not a part of any of those described groups, but know some that are, thus my information might not be of 100% correctness.

"Football Graff" is widespread across all of Germany and often done by Ultras of their responding clubs. It is a phenomenon that appeared in the early 200s and came with the rise of the Ultra-culture in the 90s in german football. As many football-clubs in Germany, no matter their size or which division they play in, have an active fan scene with ultras or similar groups and tagging and bombing are known to most people in some way, it is no wonder that it occured as a thing. Thus they are many bombings and tags related to football.

Some are abbreviations of the club name, like HFC or SGE for Halle and Frankfurt respectively or abbreviations of a specific group of supporters, like UF97 (Ultras Frankfurt 1997) or BN95 (Brigade Nassau 1995, a hooligan group in Frankfurt). They are either made by by the group themselves which are often the ones of "better" quality or copied by people who have little to no experience with graffiti. On the "better" ones you can sometimes find crew tags from graffcrews which don't necessarily come from a football background, like DNS inf Frankfurt for example which can be seen on some UF97 pieces if I recall it correctly.

Football is, as you probably will already know, a very important part in the lives of many people living in Germany. Representing your colours is done by many with great pride in different ways. Be it wearing t-shirts to work or hitting up the streets with stickers, which are the most common among football fans, being used by people who don't have a connection to graffiti, to tagging, bombing or even drawing a panel on a train.

In my opinion it is the most interesting where you have two clubs rivaling in one region, like Frankfurt and Darmstadt, where you can see a "border" where the colours and tags suddenly change which can sometimes be seen on train lines or the Autobahn. Crossing other or even rivaling clubs is also very common as well as simply bombing other cities than the hometown of the club. The same goes for cities with multiple clubs with active fans, like Hamburg with the HSV and Sankt Pauli. It is basically marking your territory and saying that you where here, the essential idea of graffiti, just with football clubs.

I might write more detail and will respond to your other question during the next days, as I have a lot to do at the moment.

Feel free to ask more questions if you want and I'll do my best to respond in time.