When a FOCUS application accesses a data source, it needs to know how to interpret the data that it finds in the file. The application needs to know about:
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The overall structure of the data. For example, is the data source a relational database, a hierarchical database, or a simple sequential file? Depending upon the structure, how is it arranged or indexed?
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The specific data elements. For example, which fields are stored in the data source, and what is the data type of each field&emdash;alphanumeric, integer, date, or some other type.
An application can obtain the necessary information to interpret a data source by consulting a description of the data called a file description. The primary file description is called a Master File. For some types of data sources these Master Files are supplemented by related file descriptions called Access Files. You need to generate one Master File&emdash;and, for some types of data sources, one Access File&emdash;to describe each data source.
Segments describe key fields and sort order, as well as the relationship of the segment to its parent.
For example, the Active Current Status (AC) file has the following file structure:
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Segment Name = ACTIVE
Segment Type = S1
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Segment Name = ACTPSN
Parent = ACTIVE Segment Type = S1
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Refer to your FOCUS documentation for detailed descriptions of the various segment types.