Disclaimer: I'm merely thinking this and am not professing any authoritative knowledge on this topic. I started this by thinking that the word complex in complex-PTSD seems out of place, but after writing it out, it actually seems pretty fitting.
I think the term complex in complex-PTSD is included because there is not one specific traumatic experience that causes the issues, but a long series of psychological stressors that amount to an entire complex of issues resembling PTSD-like reactions. What makes it more complex is that the stressors are associated with regular social interactions, allowing normal social interactions to reinforce the associated unhealthy responses that altogether amount up to a complex.
What makes the trauma-like experiences more complex is their relatively temporal distance from their consequences and that they are a series of smaller steps with the majority of them generally being understood as not traumatizing. With traumatic events in PTSD, the events and consequences happen very close in time and clearly defined. With complex trauma, there rarely is one event and the consequences of the series of events can continue to be reveals years later. With PTSD, the stressor and other relevant cues are associated with the outcome. This allows individuals to easily develop a strategy for recovery that helps extinguish the association between actually safe triggers and the outcome. In regards to making sense of the traumatizing event, it is relatively easy since people can focus on the aspects of only one or few similar events.
With complex, there is an incomprehensible series of minor events that loosely lead to the consequences, some of which have not even been recognized yet. This makes it impossible to focus on analyzing a traumatic since there are too many. Even worse, since the temporal distance between the event and the recognition of the consequences are so large, the person experiences part of the traumatic damage too separate from the causal events to make a direct association. Instead, the person learns that there needs to be extensive analysis of a series of events in order to make sense of the traumas. This creates a habit of constantly analyzing to recognize more stressors and consequences, adding a domain to complex-PTSD that PTSD doesn't have. This extra domain can cause reinforcement of the complex-PTSD as the person inadvertently but actively re-traumatizes themself by recognizing new traumas (events and consequences) in hopes to achieve safety as with all traumas. In other words, whereas PTSD reinforces itself by avoiding triggers which prevents extinction of association between them and expected consequences and thus physical and emotional reactions, complex-PTSD further grows itself by continually re-traumatizing the person long after the initial events because the traumatization happens at the realization. It's not just avoidance preventing the recovery. It's further traumatization actually making the problem worse by adding more trauma into the formula. It also creates a self-growing cycle in which analysis causes trauma which calls for more analysis and on and on.
Ok. I'm done. I may continue this process later. Thanks for reading! If you would like to join, jump in! We can even make it its own post.