Similar thing in Belgium: "stamcafé" or "stamkroeg" in Dutch. It just refers to your local pub which you (and maybe some of your friends) frequent often enough to be a regular. Not to a table. Mine isn't even local, I regularly travel 60km by train for beers and good company in a nice atmosphere.

The flag thing sounds like a very German thing to do. Maybe that's why German tourists are notorious for using towels to try to reserve chairs in tourist resorts the world over? Like draping a flag over a Stammstuhl.