In many places, though, it might not be so easy to determine what your geographic parish is. The diocese obviously knows, but parishioners? In an urban environment or an environment with a lot of Catholic parishes, sometimes it can be difficult to know where one canonical parish ends and another begins. (Even calling the parish office doesn’t always help… I once called a parish office to ask what the territorial boundaries were. The very sweet secretary told me that she had no idea, but that I was welcome to attend Mass and register there.)
Recently our diocese, which does allow the faithful to register in any parish even if they don’t live in its territorial boundaries, published on their website a nifty little map that showed the territorial boundaries of all the diocesan parishes. There were some surprises! A lot of people found out, for instance, that although they lived closer to Parish X and were registered there, their home was actually in the territory of Parish Y. The reverse was sometimes true – we were registered in one parish, slightly farther away from our home than what we thought was our geographic parish. But surprise, we actually do live in the territorial boundaries of the parish we are registered in. The map caused a lot of interesting discussions for a couple weeks on the local blogs and at the parish coffee hour…
I guess all of this is to say that in some areas, it might not be very easy for the average lay person to know whether or not they reside in the territorial boundaries of the parish they are registered in. (Nor, as another matter, do most Catholics know about the canon law articles cited here.) I imagine that’s why requests to marry outside of one’s geographic parish are so routinely approved in most places, as pastoral prudence no doubt frequently weighs in favor of approving good-faith requests.