The difference is very subtle and depends on how you frame it. For me, I think fragments of the personality or psyche move in a continuum of PARTS that goes from emotions to alters. They all are adaptive and protective.
Emotions as parts
Emotions have been explained recently in terms of adaptation, as messages that our system delivers to help us survive, avoid danger, and deal with others. Emotions alert us about situations we confront and motivate us to act according to the event. Those signals are trying to tell us that we need to be proactive because we may be in danger, or in need of action; we’d experience tightness, tingling, warmth, tears, sweat, or those butterflies in your stomach as when you see that beautiful person approaching you.
Anger may be trying to tell us we need to protect ourselves, maybe from abuse.
Envy could be trying to say that we need to work harder to reach the position we wish to attain and we haven’t.
Guilt may be saying that we should stop acting the way we have because we can damage others.
Shame may be advising us to make corrections to our actions
Anxiety must probably be saying that we need to take care of something because we may not like the consequences if we don’t.
In this case, those emotions can be interpreted as parts that can interact with our cognition in a conversation if we listen to them. If they have a message, and we focus on the message, we are actually “hearing” the emotion.
Emotional states as parts
When an emotion is not heard and doesn’t meet its function of putting ourselves into action, it stays in our system, alert for the next time we need to hear them. They become louder, more extreme, and could even influence uncontrolled behavior.
Let’s imagine the scenario where you were learning to drive, you are in front of the line on stop with the red-light and the car behind hunks. You respond with a startle and an emotion. The emotion could be of shame, guilt, anger, etc. You may not have any other option than to push the gas pedal and move. The emotion was not able to be heard and exercise its motivation for action. After years of having had the same experience when you are still at the red-light and fail to move before the guy behind honking, your emotion will be an accumulation of all those other moments in front of the light. If what you have accumulated is anger, you may yell, confront the driver, maybe curse.
That accumulation has become a more permanent part than the emotion alone. It lives in your body as a reminder of hating that experience because it makes you feel slow, criticized, pressured, called an idiot, or whatever you interpret the honk means. It has a clear behavior and will exercise its power against the will of your reason, or the opinion of other parts.
You could have long conversations with this part and you will notice how it has its own point of view that will be difficult to change.
Ego states as parts
While emotional states can be powerful but inconsequential, ego states are much more powerful and their presence in our system can have more important effects.
Emotional states were formed by the accumulation of an emotion that was not been able to do its job. Ego states form by an interruption of the emotional response because of the impossibility to do its job. Instead, it’ll create an imprint (from that age and that “reality”) of the way the response got stuck and the emotion frozen, keeping it as a reality. Instead of waiting to act out the emotion, it will create a schema of who s[h]e is depending on that emotion or emotional response.
Using the same example of the red-light and the honk, let’s imagine that the person becomes frozen after the honking and the car hits him behind; s[h]e will feel like such an idiot and a bad driver that will stamp in his brain the schema of “I’m inadequate.” The feeling of inadequacy will stay “frozen” in the system and will have an impact on many other aspects of his/her life because it’d have the conflict of whether to push the gas pedal, or to stay pushing the break.
An ego state is much stronger than an emotional state because it will affect the way the person perceives him/herself. It will create a belief about the self that needs to be disconnected from the rest because it’s shameful to be “discovered.” They may live in the person’s psyche in an unconscious way, affecting the way the person behaves. It’ll be in the background and will become alive when “inadequacy” comes out. S[h]e will freeze, feel the shame and embarrassment, will feel like hiding, quitting, etc. It will be in conflict with the system, and will live in the system but in a dissociated way.
Alters as parts
As opposed to being formed by either accumulation or interruption of an emotional state, alters form out of living an unbearable situation and finding a remedy for it. An alter has a distinctive way of dealing with intolerable pain. Basically, the psyche of a person that goes through an extremely painful event creates an alternate identity (or alter) to experience the pain in their place. This is an extremely dissociative experience but using exactly that characteristic to create the solution.
Using the example of the driver, imagine that s[h]e pushes the gas pedal after the startle and kills a pedestrian. Carrying an inadequate ego state will not be enough protection to deal with the situation. It’ll have to split further assigning the actions to a different entity.
Now to keep the memory of manslaughter functionally isolated, the person will need to reconstruct the past and creatively(and perhaps constantly) reinterpret present events in order to obscure the nature of that painful episode. That strategy seems to make the most sense when assigned to a single subject who manages the initial dissociative split, who experiences the relevant conflicts and responsibilities, and who takes steps to resolve them or pay for the actions.
They will live in the system probably as undesirable, which could even create the amnesia of the “reckless” personality or the “stupid” response. It’ll be harder to access this part for obvious reasons.
That’s how they can differ among each type of part clinically. I could go into more sophisticated explanations of parts if I consider paranormal situations, but that’s not your question.