Data Structures are used to describe all data sets processed by applications.
Data Structures are logical groups of segments, which may be:
A data structure is described logically, with no physical characteristics. This description may then be reused for different purposes : for example a given segment may be used in a database and in the programs which access the database.
Information of physical nature (such as organization, block size, description type, etc.) is added when the data structure is used in programs or database blocks.