The Type and Attribute Description of the Sequence determines whether Items are present; conditionality constraints on Data Elements of the Items cannot force an Item to be present.

Historically, many IODs declared Type 1 and Type 2 Data Elements of the Sequence to be Type 1C and Type 2C, respectively, with the condition that an Item is present. This is exactly the same as simply defining them as Type 1 and Type 2.

In particular, the conditionality constraint "Required if Sequence is sent" on the Type 1C or Type 2C Data Elements subsidiary to a Type 2 or 3 Sequence Attribute does not imply that an Item must be present in the Sequence. These conditions are meant to be equivalent to "Required if a Sequence Item is present", and the conditionality is not strictly necessary. Any Type 2 or Type 3 Sequence Attribute may be sent with zero length.

In particular, the conditionality constraint "Required if <name-of-parent-sequence-attribute> is sent" on the Type 1C or Type 2C Data Elements subsidiary to a Type 2 or 3 Sequence Attribute does not imply that an Item must be present in the Sequence. These conditions are meant to be equivalent to "Required if a Sequence Item is present", and the conditionality is not strictly necessary. Any Type 2 or Type 3 Sequence Attribute may be sent with zero length.