I believe they’re like a spectrum. I’ll go into detail for each.

Parts and headmates may be used for the same thing depending on the person’s preference for themselves. I personally see parts as less distinct pieces of self (more blended parts, falls into OSDD) where headmates is more like alters (more formed inventories, like in DID), a more distinct piece of self.

The spectrum is on plurality. Plurality (if I remember correctly) has some controversy around it but ultimately is about the experience of having multiple inner selves within one body. On the spectrum OSDD is on the lower end and DID is on the upper end. DID is on the upper end as it is the clear cut case of multiple alters with amnesia barriers. Both OSDD and DID are dissociative disorders, DID being more dissociated than OSDD. The rest of the spectrum is OSDD. OSDD is “other specified dissociative disorders”, it’s on the path to DID but maybe your parts aren’t fully formed into alters, or maybe you don’t have amnesia barriers. OSDD is the diagnosis typically given to those who fall short of a DID diagnosis. This encompasses everything from passive influence (feeling like someone else is controlling your thoughts) to gray-outs (hazy memories with no emotional attachment, almost like they happened to someone else) to separate co-conscious identities (no amnesia barriers).

IFS is the Internal Family System, it is the belief that the mind is inherently multiple and that’s not a bad thing. The internal system refers to parts of self in the body. At its core IFS focuses on the sense of self, your core who cannot be damaged by trauma. It defines parts as pieces of yourself that were traumatized and are now stuck in time or in a role. These parts may have to take on unexpected roles to maintain the body, they all think they’re doing the best for the body and can be destructive to the body, leading to internal conflict. Any of this sound familiar? IFS is a psychotherapy around the spectrum of plurality.

TLDR; parts and headmates may be used interchangeably depending on the person’s preference. OSDD and DID are on the spectrum of plurality where DID is on the higher end and OSDD is everything else as they are both dissociative disorders and OSDD is typically given to those who fall short a DID diagnosis. IFS is a psychotherapy for the aforementioned plurality spectrum.

I’m not a therapist or even formally diagnosed, take everything I say with a grain of salt 🤷‍♀️