Physicians use the DSM-V (a book) to diagnose mental health conditions which works on a checkbox of symptoms basis and is, thus, open to error/mis-interpretation. Those diagnoses then provide prescribing guidelines for medication/treatments.
Us psychotherapists only use the DSM to ensure a GP hasn't missed a condition that requires psychiatric treatment rather, or alongside, psychotherapy.
Just thought this may help explain the vagueness and clashing diagnoses sometimes experienced.