The Purpose column contains one or more words describing the role played by the dataset in the message. The Level column is the key to describing the hierarchical structure. Instead, the number of left parentheses indicates the segmentation depth within the message at which a dataset is allowed. There are no other fields in a segment end line. It is not permissible for two members or subsegments of the same segment to have the same dataset category in the Name column, nor may a message have multiple level zero entries with the same Name.
Segments are always repeatable in messages.
In essence, it looks like the image of an actual dataset from some message families, with a presence letter replacing each main field. A field that is optional for a dataset according to its DSSL specification may be declared mandatory in the context of its use in a particular message type. The specification of constraints on messages is quite similar to the specification of constraints on datasets. In message constraints, such names refer to a zeroth level dataset of the message being constrained. In dataset constraints, expressions that refer to optional fields can be used as predicates for testing the presence of those fields.