All citizens without exception, have a cortical stack implanted in their spine at one year old. This stack becomes the receptacle for DHF (Digital Human Freight, the human mind, or 'soul.') In the event of death of the sleeve (body), the stack can be spun-up (ie, implanted) into another sleeve.
A person's stack encoding is linked to the DNA of their current sleeve. Your DNA is linked to your bank account, so scanning your DNA is the primary means of payment and identification (a very sophisticated version of tap-to-pay.) Though, the poorest and those who don't want to be tracked tend to deal in trade and hard currency. A person only needs to touch a surface for their ID to be ascertained. Different agencies/retail outlets can glean differing levels of information from this interaction depending on requirements.

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Stacks were originally developed for interstellar travel and created with an alien metal. Over time, it was developed to bestow a form of immortality on certain members of humanity. Death of bodies became akin to property damage, although it's common for most people to wear just two sleeves before they opt for storage. Despite the fact that stacks have been in use for hundreds of years, humanity does not know all that much about the mechanism that makes it possible. It is salvaged alien technology, and there are things that humanity just doesn't fully understand.

Adjusting to a new body is difficult psychologically and physically. Most people experience sleeve sickness of varying degree of severity and duration. Some people are more naturally able to handle changing sleeves, while others train themselves mentally to be able to better handle the shock. There also exists some biochemical manipulation to the stacks themselves in order to stave off sleeve sickness and personality fragmentation. Soldiers used for off-world wars via needlecasting are especially adept at donning new sleeves for short or long term missions with few side effects.

Wearing a different sleeve for a short period of time (days to weeks) is far less damaging than adjusting to a new sleeve long-term. The reasons for this are not well understood. It means those who use sleeve changing for work or for pleasure, or to needlecast short-term off-world can resleeve many times without many ill-effects - assuming they are psychologically strong. In contrast, someone wearing their birth sleeve, then another for a lifetime, often find it incredibly difficult to adjust to a third. There is some suggestion that the human mind is simply not designed to live multiple lifetimes in different bodies.

Cross-sleeving (aka, wearing a sleeve that is not of your birth/identified gender) can be especially difficult for some to handle either in the short or long term. It can lead to premature personality frag and prolonged sleeve sickness, especially if the individual is untrained and unprepared to handle it.

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Meths avoid sleeve sickness and personality frag by inhabiting clones of their birth sleeves and employing technology that makes being spun up far less traumatic. By doing this, they suffer little to no side effects from wearing multiple sleeves and living multiple lifetimes. Many meths are hundreds of years old.

Stacks can also be spun-up in VR. This is often used for psychosurgery (repairing the psychological damage from a traumatic sleeve death, from torture, or personality frag) or to question the victim of a crime. People can also jack in to cyberspace via their stack.

Society as a whole accepts stacks as a fact of life. The exception are the Neo-Catholics (Neo-Cs) who don't believe that the stack holds the human soul. They have religious coding on their stacks that prevents them from being spun up again in the event of sleeve death. This is currently contentious because someone with religious coding could not be spun up to testify against their murderer.

TECHNOLOGY

People change sleeves one of two ways: either the stack is removed from the spine and physically implanted into a new body, or short-range needlecasting is used to move the DHF into a new stack that already resides within a new sleeve. The second is far less traumatic, though considerably more expensive as it requires specialized equipment. Medical knowledge is required to remove a stack from a body without killing the sleeve, so most stack removal of this kind happens after the sleeve is already dead.