It might be easier to think about if you do away with the label altogether and do a bit of research about "structural dissociation".
Here's what the guys who are on the comittee for reviewing the dissociative disorders for the DSM-V are saying:

In the theory of structural dissociation the personality fails to integrate during early childhood, due to a lack of appropriately nurturing experiences. Basically the theory states that the functional part of the personality (ANP) and the emotional part of the personality (EP) never learn to work together as an integrated system. Can be lots of reasons for that... constant invalidation or unpredictability from primary caregivers, ongoing abuse etc etc... the basic premise is that a young child never learns (read: is never taught) to regulate or soothe their own emotions.

So... the ANP and the EP operate independently from one another, often in denial of one another, like the black/white sorta thing gonig on in BPD. ANP denies the reality of the emotion (EP) and the EP feels it all and continues to feel invalidated, but this time by the self (ANP) I dont know if i make any sense, haven't slept in over 50 hours)

Anyway, that is called primary structural dissociation - that basic split between the feeling self and the fucntional self. Not amnesiac from eachother, but those two splits can form a self-identity... like I know this is me (ANP), but when I am like THAT (EP) that is NOT me.

So secondary structural dissociation is when the EP personality split divides into secondary splits in order to cope wtih different intense emotions. Like.... the original EP might react to traumatic situations by freezing (a common physiological response to trauma, and becomes a conditioned response of the original EP through repeated traumatic episodes.), but then a new ongoing traumatic situation might start occuring in which the best adaptive response of the child is to fight. the ANP part can't do that (never learned to cope with trauma), the original EP is conditioned to freeze, and yet another part of the brain is activated into the fight response. If that experience is also repeated over time, the fight response will also become a conditioned physiological response to that particular trigger, thus two separate emotional states (EPs) will exist.

Tertiary structural dissocation is when the ANP part also splits into several parts to avoid triggers that cue the EP's. the ANP's job is to avoid the trauma (EPs) in order to maintain daily life (as a child, being 'okay' or pretending nothing is wrong in order to gain necessities of life from caregivers, and as an adult, working, studying, having relationhips, having a family - also in order to gain the necessities of life). So the ANP is phobically avoidant of the EPs. In childhood there were obviosuly circumstances that made it not safe to be for the EP, and in adulthood being in touch with those feelings could overwhelm and mess with essential daily tasks like going to wrok or parenting.
Sooooo. the ANP splits, to avoid triggers. Like.... if being intimate with one's partner is too close to the trauma and triggers EP stuff, an isolated ANP part night evolve to contain that trigger ... so the main ANP gets to remain blissfully unaware of the EP crap.

Because... basic reason is ANP can't deal with it. Never learned to deal with those emotions as a child (not safe to) and still doesn't know how to. Any slight little bit of those EP feelings threaten to completely overwhlem the ANP becuase the ANP has no coping resources to deal with them. So ANP likes to stay in la-la land. nothing bad, here, no sirreee. ANP doesn't think about it. And when Ep has been triggered into the front for a while, when ANP gets back in town she sure as heck doesn't want to admit that anuthing was wrong. uh uh.

Anyway... some researchers say primary SD is kinda like BPD and PTSD. And secondary SD is like complex PTSD. And Tertiary SD is like DID.

Who knows. Its just a theory. It makes sound biological physiological and emotional sense to me, but i am pretty sure i just rambled and confused the heck outta ya'll.
look it up. Interesting stuff.