A level concept or hierarchy is inherent in the structure of a logical data record. This concept arises from the need to specify subdivisions of a record for the purpose of data reference. Once a subdivision has been specified, it can be further subdivided to permit more detailed data referral.
The most basic subdivisions of a record, that is, those not further subdivided, are called elementary items; consequently, a record is said to consist of a sequence of elementary items, or the record itself can be an elementary item.
In order to refer to a set of elementary items, the elementary items are combined into groups. Each group consists of a named sequence of one or more elementary items. Groups, in turn, can be combined into groups of two or more groups, and so on. Thus, an elementary item can belong to more than one group.