Our particular complexities would be our own, but like you suggested, it can be hard to untangle. I have traumas from very early (very serious medical, hospitalized away from family), traumas I don't remember, traumas I remember in snippets or still shots, or just feel in my body.
Complex refers to multiple traumas, over a period of time, so for kids in their developmental years, that can encompass a range of other problems that don't fit just PTSD...personality, somatoform, dissociative stuff gets in here. The symptoms can be far ranging. So the symptoms are complex, the diagnosis might be complex, and the treatment is complex (though not so much if you have therapist willing to treat a case of complex trauma, even if it's not in the DMS V). But for example, without the "complex" part, I could easily have PTSD with avoidant personality and somatization disorder and probably depression....maybe even ADHD. My therapists see this all through the complex trauma lens. My particular symptoms are very somatic (anorexia, panic attacks, chronic pain, etc). But the somatic pain is complex in itself since some is trauma stuff and others are just long-lasting ways of "feeling" when I can't have emotions. I have physical sensations instead. I feel pain instead of bad emotions. My pain is containing stuff I don't know how to deal with. For another C-ptsd person, they might be dealing more with dissociation. Because of so many different traumas, especially if they happened in early years, it simply becomes "complex" because we present with what seems like seven different major problems.
I don't have flashbacks except for recent, adult traumas. My worst traumas were just encoded somewhere in my body and have somehow also shaped my personality. I wish they'd figure out the labels better. I'd relate better to C-PTSD better than PTSD with a side of pain disorder. C-PTSD seems more comprehensive of somatoform and dissociative issues because we pick those up through enduring traumas, not typically one-time traumas. They become a way of living.
Not sure if I answered your question. But I do see a difference and it would be helpful if the people in charge of the DSM could get it together so we could move along with differentiated treatment