The video above shows some of the manufacturing processes of making Hornady bullets and ammo.
Brass, the most commonly used cartridge case (at least here in the US) is an alloy of Copper and Zinc.
In the video they talk about Annealing the brass. This is when the metal is heated in order to make it more flexible and softer so that it is easier to work with. The more brass is worked with and shaped, the more brittle it becomes.
Annealing softens the brass and makes it easier to work again. This process is often done near the neck of the case. Reloaders may find that they need to anneal the case again after shooting the same brass a few times. You can tell when a case has been annealed as there is a discoloration near the neck of the casing.
The above round of 62gr 5.56 RORG (Royal Ordnance Radway Green), manufactured for the British Military shows distinctive annealing around the neck of the case.
Cartridge cases that have not been annealed are more likely to experience splitting near the neck. I’ve seen this many times with older 8mm Mauser ammo which had clearly never been annealed.