A line always contains a hierarchical level number, a tag, and an optional value. A line may also contain a cross-reference identifier or a pointer. This means that the data is self-defining. Tags allow a field to occur any number of times within a record, including zero times. The hierarchical relationships are indicated by a level number.
Subordinate lines have a higher level number.
A line and its sub-lines constitute a context or enclosure, that is, a cluster of information pertaining directly to the same thing. This hierarchical arrangement corresponds with the natural hierarchy found in most structured information. A series of one or more lines constitutes a record. The cross-reference identifier is analogous to a primary key in relational database terminology. Elements may be described in terms of a set of other elements, some of which areselected from a set of alternative elements.