The arrows indicate the air gap in this brake rotor. This is a critical dimension. Cheap, inferior rotors will have a wide air gap, as less steel is needed for a rotor with a wide gap. For a manufacturer, less steel means significant savings over millions of brake rotors. For a Corvette owner, though, this is a problem. The rotor won't have enough steel to properly absorb the heat from stopping. It also means the rotor does not meet original equipment standards. If you're buying new rotors to replace your originals, compare the measurements. If a shop is doing the work, explain that you intend to compare the measurements before you pay the bill. You'll need to say this nicely, though, or you might have to find a new shop (which may not be a bad idea if they get upset over your knowledge of brake rotors).