Which part does the "wearing"?

General question about flat-tappet cams... during the break-in period is it the lifter faces or the cam lobes that wear off? Whenever i hear talk about proper break-in it sounds like both parts wear together but then that wouldn't make it possible to use new lifters on an old cam right? Also the cam is made from cast iron and the lifters from steel i'm guessing the cam is the harder of the two?