What is the difference between DID and Tulpas?
For one, they're different categories of things. DID is a disorder that causes or affects plural systems and tulpas are a type of headmate in a system.
("Headmate" is catch-all term that can apply to any autonomous agents in your head, whether a tulpa, an alter, or something else entirely.)
A tulpa is a specific type of headmate created by repeated interaction. This can be a headmate that starts with only basic traits and grown out from that, or it can be a headmate based on a fictional character that the creator interacted with a lot with in their mind.
DID is a disorder that virtually always stems from trauma. Usually, the first headmates in DID systems are believed to be created by dissociation. (Existing headmates don't associate traumatic experiences as belonging to them, and those memories are used to create new alters. Not all headmates form from trauma though, and there are cases of DID systems reporting being plural before the trauma that caused the disorder.) Headmates in DID systems often find it difficult to control switching, with the process becoming an instinctual reaction to cope with trauma, and memory barriers are created to block out those traumatic experiences. There is also a myriad of (c-)PTSD symptoms that come with that.
Tulpas in non-DID systems will have an emotional memory separation where they will associate certain memories as being theirs and certain memories as being someone else's, but there aren't any amnesiac barriers unless intentionally built through an incredibly involved and intentional process. Switching is possible and even common in non-DID tulpa systems, but it's usually a voluntary skill that has to be learned, not something that is done instinctually.
Both categories do experience multiple autonomous agents internally, and given that I know of multiple DID systems who have created alters by following tulpamancy guides, there is a certain link between alters and tulpas. Tulpas created in DID systems will often follow the same "rules" as other headmates in the system, being subject to inter-identity amnesia and experiencing instinctual switching.
There are probably a lot of points I'm missing, but at the most basic level, those are the main distinctions and similarities I've observed.
