Soulbonding is typically seen as a spiritual occurrence where a character from fiction essentially bonds to a person. Soulbonds are souls from another universe or plane of existence that "latch on" to their Host. That other plane of existence is usually the universe of a fictional show or place, but it can be a universe the host created as well. The soul usually already has a form, identity, vocality, etc. because they already developed those things in their own universe. In other words, soulbonds are like walk-ins. The term was coined by Amanda Flowers of the Just For Writers Mailing List.
A soulbond functionally can be seemingly sentient from the start and autonomous unlike a tulpa (created from scratch), but may not see themselves as a form of plurality. Of course, the person can have multiple soulbonds from various tales of fiction similarly to Tulpamancy, and there is no limit to the amount of soulbonds a person can have.
Not all soulbonds will live with the host. Lots of soulbonders have long-distance soulbonds, which are soulbonds who stay in their universe but can still be communicated with. Other soulbonds, called day-trippers, will go back and forth between their universe and yours. Permanent residents are soulbonds who permanently live with the host.
Soulbonding is much more frequent to creators of worlds, especially authors having soulbonds of their own characters. There have been studies of authors, which the study stated that over 90% of authors have experienced their characters acting somewhat autonomously(with the freedom to act independently). For the majority, soulbonding in itself is an unintentional process unlike Tuplamancy, hence why the community views it as such a specialty. However, it isn't always unintentional. Lots of people will deliberately soulbond with a character, by indulging in media to do with that character, calling out to them, and much more.
In present day, soulbonding is much less common. The separate soulbonding community has gone quiet due to many non-soulbonding systems having fictives, which are different from soulbonds, and can have a variety of akiness to characters, but don't always view themselves as full real characters, of which soulbonds see themselves as.
Distinguishing From Other Experiences
Mental disorders: Dissociative Identity Disorder (DID) and Other Specified Dissociative Disorder (OSDD-1) are post-traumatic mental disorders that cause pathological multiplicity. It's not uncommon for DID and OSDD-1 systems to mistake their pathological experiences for something non-pathological, such as imaginary friends, spiritual possession, soulbonds, or other forms of non-pathological multiplicity.  This is because amnesia of childhood trauma is very common. Furthermore, many pathological systems are not distressed or even aware of their dissociative symptoms until later in life.  This is why it's very important to talk to a professional if you believe you are experiencing multiplicity in order to rule out pathological causes!