On my Mavic rims, there's a groove about 6mm from the circumference, maybe 1mm deep. Itís on both sides of the rim. The groove bisects the plane that the brake pad contacts the rim when brakes are engaged.
All I can make of this is that itís a rim wear gauge. If so, it just seems that thereís a whole lot of potential brake friction acreage taken up with ďmereĒ measurement.
So. . .what IS this thing?
Anyway, Iíll bet someone out there in BF Land knows thí answer. . .as usual Mercy Buckets [as my ancestor Clovis said when the Bloody Revolutionaries spared him from the Gilly-Tene].