What are the "stacks" in Altered Carbon?
Those digitized souls we mentioned? Those are called "stacks" -- short for “cortical stack" -- which are implanted into biological humans when they're 1-year-old. Stacks contain the human mind and everything that comes with it, including memory and emotion. Stacks essentially make their owner immortal, as they can be endlessly re-uploaded into new bodies so long as the stack itself isn’t damaged.
As we learn, certain groups, like Catholics, resist cortical stacks so they can get into heaven, which is impossible if the soul never truly dies. Because of their beliefs, Catholics are targeted by criminals since they won’t be around to identify their assailant.
When a stack is permanently destroyed, it is referred to as "RD," or "real death."