This article refers to the International Classification of Diseases 11th edition (ICD-11) which is the official classification system for mental health professionals working in NHS clinical practice. The literature occasionally refers to the Diagnostic and Statistical Manual of Mental Disorders (DSM) classification system which - whilst used in clinical practice in the USA - is primarily used for research purposes elsewhere.
This is a companion discussion topic for the original article at https://patient.info/doctor/nightmare-disorder