Table 4.

Attribute Types.

Type	Description
Type 1:	Required to be in the SOP Instance and shall have a valid value.
Type 2:	Required to be in the SOP Instance but may contain the value of “unknown”, or a zero length value.
Type 3:	Optional. May or may not be included and could be zero length.
Type 1C:	Conditional. If a condition is met, then it is a Type 1 (required, cannot be zero). If condition is not met, then the tag is not sent.
Type 2C:	Conditional. If condition is met, then it is a Type 2 (required, zero length OK). If condition is not met, then the tag is not sent.

The table displays Attribute Types as defined in the DICOM standard.