There has been some confusion around this topic, even among professionals; since the Complex PTSD diagnosis has been established, they have found that some persons who have been diagnosed with borderline may better fit the construct of CPTSD; however, it is not correct to suggest that they are identical or interchangeable. Here's a recent study that worked towards more precisely identifying the divide: Distinguishing PTSD, Complex PTSD, and Borderline Personality Disorder: A latent class analysis