"Your body is not who you are. You shed it like a snake sheds its skin. We transfer the human consciousness between bodies to live an eternal life".
Sleeves are the term used for bodies that serve as a receptacle for a cortical stack, whether they be naturally born, genetically or cybernetically modified, or artificially created, to augment their user's physical or mental abilities, as well as grant new abilities altogether.
Once the use of the tech known as cortical stacks became ubiquitous, human bodies were considered no more than "sleeves" for the human consciousness. The body an individual is born with is called their "birth sleeves". Sleeving is a process where an individual's consciousness is transferred to another body and was developed with cortical stacks as a method of interstellar travel, by transferring the human consciousness to worlds that are trillions of miles away in seconds to a waiting sleeve, called Needlecasting.
Over time, sleeving became a method for extending life indefinitely for those who could afford it. Ordinary citizens can transfer to a new sleeve after death but as the poor cannot afford sleeves of the highest caliber in terms of age, health, etc. they are left purchasing whatever bodies they can afford or are empty and available regardless of their condition, so most citizens re-sleeve at least once after death then remain in storage. There is a legal precedent called "victim restitution" which provides a free sleeve to the victims of accidental deaths but, like all citizens, they are provided whatever empty sleeves are available with younger healthier sleeves requiring a payment most cannot afford. The wealthy however can afford to have sleeves designed to their specifications, often growing clones of their original bodies to transfer into. It is stated that a single clone costs more than an average citizen makes in a lifetime.
Sleeves can be altered or produced with genetic and cybernetic enhancements for physical and mental augmentations, superior in strength, speed, reflexes, stamina, endurance, and senses. This can be seen when Reileen breaks into Fightdrome, as she is able to perform a massive leap effortlessly. Such modifications can also come in the form of neurochem and other forms of drugs and enzymes designed to alter emotional and mental capabilities. The military branch of the Protectorate uses augmented "combat sleeves" for its soldiers, which have a more potent, military-grade version of neurachem, as well as combat muscle memory conditioned into them. As an alternative to organic sleeves, citizens of the protectorate can choose to re-sleeve into synthetic biorobotic bodies of varying designs, though most choose not to as synthetic sleeves can only approximate the physical sensations organic sleeves naturally posses. Therefore, some who use synthetic sleeves can lack physical sensations or senses like taste and smell.
When an individual's birth sleeve dies or a new sleeve dies violently, they can often experience an intense trauma when placed in a foreign sleeve. The more foreign the sleeve in terms of age, race, gender, etc. and the more violent the death, the more intense the shock. Sleeving facilities often have therapists and psychologists on hand to assist with these transitions. Transferring to a sleeve that is closer to an individuals birth sleeve, i.e., a clone, helps avoid this effect. There are mental conditioning techniques that are capable of preventing sleeve shock; as seen with the Envoys, a group of elite super soldiers trained to withstand the psychological trauma that comes from frequent re-sleeving.
When an individual is re-sleeved into a previously used Sleeve, even ones placed in storage, the sleeve still produces pheromones and hormones and maintains the "memories" of the sleeve's previous users physical activity, experiences and even cravings. When Takeshi Kovacs was re-sleeved into the birth sleeve of Elias Ryker the sleeve maintained Ryker's nicotine addiction and Takeshi began smoking cigarettes even though he himself hated them. Takeshi also developed a strong attraction to Kristin Ortega based on her previous relationship with Ryker, stating the attraction was based on his sleeves "memory", i.e., the pheromones between Ryker's sleeve and Ortega's.
With the digitization of human consciousness there arose the possibility of copying a consciousness similar to how a computer program is copied, creating a perfect duplicate of a persons mind. Creating a duplicate of oneself within another Cortical Stack is not inherently illegal but placing a duplicate consciousness in a sleeve is called "Double Sleeving" which is highly illegal due to the problematic nature of creating duplicates of oneself and is punishable by real death.
When an individual is re-sleeved into a sleeve of the opposite gender to the one in which they were born. In Fallen Angel, Takeshi Kovacs attempts to learn more about Lizzie Elliot by going to Jack It Off, her previous place of employment, and speaking to a worker who knew her. He only gets information by pretending he is Ava Elliot, Lizzie's mother, cross-sleeved into a male sleeve after serving her prison sentence for her work as a Dipper.