The extreme left side of a Roper map island would be the left-most point of that island where a rope can be attached (while still allowing for the trick in question to be done). On the right one of the two islands on Vahagn's NR5 map, the extremes would be these two points:
The definition of an Arch would then be this same trick, except done between two rope attachment points that
aren't the extreme points of any island. Then you just need to modernise the definition of Mexi to include flips across more than a single island and make an exception for "islands" that are simply poles, like for example the four little things sticking from the ground on
this map. Those would be Arches even though technically, the poles are islands too.
And then you're pretty much set, as long as you don't expect roping to be an exact science. In practice, everything is more a matter of degrees and momentary inspiration, like Anubis points out, but I think it's also nice to have the community agree on the theory behind it all, just so roping discussions can be a bit more clear.