The C means "complex". Many psychologist/specialists have wanted a separate diagnoses of complex PTSD because, while the symptoms can be the same as PTSD that developes from experiencing a trauma, a person can develop PTSD from experiencing several traumas, even traumas that occured during their childhood which the therapy is going to be more complex simply because the challenges an individual may have can be more complex.

Every once in a while I post this Link, it's worth reading through and it does talk about Complex PTSD.

Complex post traumatic stress disorder (complex ptsd, pdsd, shell shock, nervous shock, combat fatigue), symptoms and the difference between mental illness and psychiatric injury explained