PS3.3 of the DICOM Standard specifies a number of Information Object Classes that provide an abstract definition of real-world entities applicable to communication of digital medical images and related information (e.g., waveforms, structured reports, radiation therapy dose, etc.). Each Information Object Class definition consists of a description of its purpose and the Attributes that define it. An Information Object Class does not include the values for the Attributes that comprise its definition.

Two types of Information Object Classes are defined: normalized and composite.

Normalized Information Object Classes include only those Attributes inherent in the real-world entity represented. For example the study Information Object Class, which is defined as normalized, contains study date and study time Attributes because they are inherent in an actual study. Patient name, however, is not an Attribute of the study Information Object Class because it is inherent in the patient on which the study was performed and not the study itself.

Composite Information Object Classes may additionally include Attributes that are related to but not inherent in the real-world entity. For example, the Computed Tomography Image Information Object Class, which is defined as composite, contains both Attributes that are inherent in the image (e.g., image date) and Attributes that are related to but not inherent in the image (e.g., patient name). Composite Information Object Classes provide a structured framework for expressing the communication requirements of images where image data and related data needs to be closely associated.

To simplify the Information Object Class definitions, the Attributes of each Information Object Class are partitioned with similar Attributes being grouped together. These groupings of Attributes are specified as independent modules and may be reused by other Composite Information Object Classes.

PS3.3 defines a model of the Real World along with the corresponding Information Model that is reflected in the Information Object Definitions. Future editions of this Standard may extend this set of Information Objects to support new functionality.

To represent an occurrence of a real-world entity, an Information Object Instance is created, which includes values for the Attributes of the Information Object Class. The Attribute values of this Information Object Instance may change over time to accurately reflect the changing state of the entity that it represents. This is accomplished by performing different basic operations upon the Information Object Instance to render a specific set of services defined as a Service Class. These Service Classes are defined in PS3.4 of the Standard.