DICOM Attribute Requirement Types
| 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. |