The relationships among all data to be used in a program are defined in the Data Division, through a system of level indicators and level-numbers.
A level indicator, with its descriptive entry, identifies each file in a program. Level indicators represent the highest level of any data hierarchy with which they are associated; FD is the file description level indicator and SD is the sort-merge file description level indicator.
A level-number, with its descriptive entry, indicates the properties of specific data. Level-numbers can be used to describe a data hierarchy; they can indicate that this data has a special purpose, and while they can be associated with (and subordinate to) level indicators, they can also be used independently to describe internal data or data common to two or more programs. (See "Level-Numbers" in topic 5.3.4 for level-number rules.)
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