That's a case separation sign. I don't know exactly why but some cases separate about 1/4 inch below the shoulder and others separate about 1/4 inch above the extractor cut. I do know to watch for them and stop reloading those cases. The next loading may only develop a split partially along that line or , it could, split all the way around and leave the front part of the case in the chamber! Bad!
From the looks of that brass it's been reloaded a few times. Time to put those aside.
Brass has a limited life so just watch for it.
Lighter loads, with cast bullets and neck sizing will extend case life some, but it's going to wear out.
Gary