Chronic PTSD means that the symptoms are present for longer than 3 months, as opposed to acute PTSD which lasts less than three months.

Complex PTSD is a different animal, in that it includes regular PTSD symptoms with more pronounced dissociation/dissociative episodes and often some traits of one of the personality disorders, often BPD. Complex PTSD occurs more with prolonged or more severe trauma in childhood.

Complex PTSD isn't in the DSM currently, so some therapists do not diagnose it or are not familiar with it. Over the years I have had therapists use DDNOS and Complex PTSD fairly interchangeably, but I think that's more for convenience than accuracy. It's all alphabet soup, anyway.