Two types of data are processed by a COBOL compilation unit:
data stored on some external medium
data generated internally during program execution.
The first type of data is combined into records in files; the second type of data must be declared by the user as records or subordinate data items.
To ensure maximum independence of data from its specific representation on external volumes and in data processing systems, the properties or contents of data are described with respect to a standard data format rather than a system-oriented format.
The data description method used allows a distinction to be made between the physical and system-dependent properties on the one hand, and conceptual characteristics on the other.
Physical and certain system-specific properties of data on an external medium are defined in a COBOL compilation unit in order that efficient use may be made of special techniques.
The term "physical properties of data" refers to
the grouping of logical records within the physical boundaries of the external volume
the recording mode in which the data is stored on the external volume.
The term "system-dependent properties of data" refers to the description under which a file is identified on some external medium.
The conceptual characteristics of data on an external volume pertain to the logical units of data, the logical records, and are not associated with any physical or system-specific properties.
These characteristics are described for files in the file description entries and for records in the record description entries.
A data description in a COBOL compilation unit is separate from the declaration of execution procedures. This permits the programmer a great number of options for modifying a data description entry without any change to the procedures which are related to that entry. Therefore, to a certain extent, the procedures of a COBOL compilation unit may be seen as data-independent.