WRITTEN BY BUTTERNUT / SHRIMP SYSTEM
Integration is the process of amnesia barriers lowering due to understanding more about your system and being away from a traumatic environment. The more you understand and the safer you are, the less need there is for amnesia, since the amnesia is a strategy to keep away negative, traumatic, unwanted memories. When integration occurs, memory between alters also gets better.
Fusion is what can come after parts have integrated. After amnesia barriers have lowered between alters, sometimes alters can can fuse. Some people claim that fusion can be a choice, while others don’t have that experience. It’s hard to say if it is true that you can actively choose for specific parts fo fuse, as there is little research on the topic.
Final Fusion is when the system collectively agrees to word towards having all alters fuse into one alter. This doesn’t guarantee that new alters won’t show up in the future.
Functional Multiplicity is when you choose to not to take the path of Final Fusion, but instead, choose to just Integrate as a collective. Fusion can and will still happen, even if it isn't the goal.
It can be hard to tell if parts have integrated or fused, but I will say, at least for integration, usually you can tell if parts have better memory and communication between each other.