Levels of data
After a record has been defined, it can be subdivided to provide more detailed data references.
For example, in a customer file for a department store, one complete record could contain all data that pertains to one customer. Subdivisions within that record could be, for example, customer name, customer address, account number, department number of sale, unit amount of sale, dollar amount of sale, previous balance, and other pertinent information.
The basic subdivisions of a record (that is, those fields not further subdivided) are called elementary items. Thus a record can be made up of a series of elementary items or can itself be an elementary item.
It might be necessary to refer to a set of elementary items; thus, elementary items can be combined into group items. Groups can also be combined into a more inclusive group that contains one or more subgroups. Thus within one hierarchy of data items, an elementary item can belong to more than one group item.
A system of level-numbers specifies the organization of elementary and group items into records. Special level-numbers are also used to identify data items used for special purposes.