For all official purposes--including insurance claims--psychotherapists are required to use the diagnoses in the Diagnostic and Statistical Manual of Mental Disorders, currently in its fifth version (the DSM-5). Unfortunately, the DSM is a limited and even inaccurate text that in some areas reflects social and cultural biases. It has been written and revised by a team of psychiatrists who are mostly male, even though the majority of mental health clinicians are female and aren't psychiatrists. I and many other therapists find it to be of limited usefulness for diagnosis and assessment.
The DSM diagnoses were developed using the "medical model," which means they are symptom-based. The problem with this model is that although it describes what disorders look like, it does not describe their etiology--how they came to be. Although the DSM includes information about prevalence and prognosis, it doesn't describe how and why disorders develop.
The reason the DSM categories and descriptions are symptom-based is because that's what insurance companies and pharmaceutical companies want--they want to know how many symptoms a patient has because this can be fairly easily measured. Non-medical therapists and psychoanalysts are more interested in the underlying psychological structures and dynamics of a patient's illness, because our goal is not just symptom reduction, which can be temporary, but lasting change.
Perhaps the best example of a discrepancy between the DSM and other models are the different conceptions of Borderline Personality Disorder. The DSM description is a grab bag of symptoms that can also be found in people with Post Traumatic Stress Disorder. This can lead to diagnostic confusion. Psychoanalytically-oriented therapists, on the other hand, assess Borderline Personality Disorder based not just on symptoms but on factors such as which psychological defenses the patient uses, what type of interpersonal relationships they have, and how they manage stress. Ultimately this type of assessment allows us to differentiate between BPD and PTSD, to tailor treatment more effectively, and to better predict outcomes (prognosis).
Cultural biases are also found in the DSM. The criteria for Anti-Social Personality Disorder (sociopathy) include law-breaking. Whose laws? Laws vary from country to country and from state to state. Some laws may be unjust. This DSM criterion equates conformity and obedience with mental health. A psychoanalytically-oriented therapist is more interested in other criteria such as the patient's capacity for empathy, presence or lack of deceit, personal responsibility, presence or absence of aggression/sadism, and capacity for trust. The DSM criteria also do not address the fact that certain groups are more likely to come in contact with the police for reasons that have nothing to do with their personalities.
Similarly, the DSM criteria for Histrionic Personality Disorder include "consistently uses physical appearance to draw attention to self." Women are encouraged to use physical appearance to draw attention to self, in Western cultures. This may have resulted in more women than men being diagnosed with HPD, although there is no evidence that women are more likely to have this disorder.
The influence of pharmaceutical companies is well-known among clinicians. A disorder originally called "Minimal Brain Dysfunction" was re-labeled as "Attention Deficit Hyperactivity Disorder" by the pharmaceutical industry, in order to better market their drugs. Many patients are unaware that "ADD" as many people call it, is a term created by entities with ulterior motives. Again, the listed symptoms of ADHD can overlap with other conditions, especially abuse and neglect of children. It's easier for a doctor to write a prescription than to call child welfare or to recommend parenting classes.
It's been said that "the winners write history." They also write the laws, the rules, and the diagnostic criteria.