The following is reproduced from the Introduction to DC:0-3R:
For the practicing clinician, the diagnostic process is ongoing. One does not make a diagnosis on the basis of a onetime "snapshot" of symptoms, but rather collects information over time in order to understand multiple aspects of presenting problems as well as variations in adaptation and development that reveal themselves on different occasions and in different contexts.
The diagnostic process consists of two aspects: the classification of disorders and the assessment of individuals. We classify disorders, not individuals. We classify disorders primarily so that professionals can communicate clearly about descriptive syndromes. Clinicians can then link their observations to a growing body of knowledge concerning etiology, pathogenesis, the course of a disorder, and expectations concerning treatment. Using the common language of a diagnostic classification system facilitates the connection of individuals to existing services and thus can aid in the mobilization of appropriate systems of mental health care. The assessment of individuals, however, necessarily precedes classification. Assessment and classification are both used in clinical formulation.
Clinical formulation involves the drawing together of multiple observations and sources of information about the individual within a general diagnostic scheme, so as to guide the clinician about what to do next. It is noteworthy that as DSM and ICD classification systems have evolved to be multiaxial, clinicians have used them not only for classification of disorder but also as guides for assessment and clinical formulation.
The first three axes of these systems deal with the classification of disorder, and the fourth and fifth axes deal with the assessment of individuals in context. DC:0-3 and DC:0-3R follow a similar multiaxial scheme.
Axes I: Clinical Disorders, II: Relationship Classification, and III: Medical and Developmental Disorders and Conditions deal with the classification of disorders. Axis IV: Psychosocial Stressors and V: Emotional and Social Functioning reflect the assessment of individuals in context.
ZERO TO THREE. (2005). Diagnostic classification of mental health and developmental disorders of infancy and early childhood (rev.). Washington, DC: Author.