Odd rear disc cutout

[blueSup]

Nothing like a good tub to help find anything that might be (about to go) wrong with a car.
On the last wash I noticed something about the rear discs on each side of the car. There's a cutout in them, and it's on both sides of the interior of the disc.
Any ideas what it might be for? It's painted from the factory so must be done on purpose.
Has anyone else noticed them?

[Umai Naa!!]

That's normal. Most brake discs these days have it.

I'm pretty sure it's to do with the casting process.

[putney40]

That's normal. Most brake discs these days have it.

I'm pretty sure it's to do with the casting process.

During production and testing they have removed metal to balance the rotor.

Nothing to worry about.

[brad]

as above. balance

[blueSup]

Interesting. I'll have a closer look at the fronts to see if they are the same.

[blueSup]

And just like that, here's the same in the front. Thanks for the info guys!