The most common reason appears to be inadequate diagnostic coverage. A substantial number of patients fail to meet the existing diagnostic criteria for one of the existing anxiety, mood, eating, or personality disorders. 

University of Illinois at Urbana-Champaign

NOS is generally used when a clinician has determined that a mental disorder is present, but the person fails to meet the criteria for one of the existing diagnostic categories. 

Not otherwise specified (NOS)

Widiger, T. A., & Samuel, D. B. (2005). Diagnostic categories or dimensions? A question for the Diagnostic and statistical manual of mental disorders--. Journal of abnormal psychology, 114(4), 494. https://doi.org/10.1037/0021-843X.114.4.494

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