SAP ELEMENTARY TYPES AND STRUCS documentation, setup help and example usage

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Data Types in ABAP Programs
ABAP offers a range of predefined elementary data types .
In addition to the predefined data types, ABAP also provides two mechanisms for structuring complex data types :

Components of any type can be grouped together in structures .

Internal tables can be defined using lines of any type.

The options "structure" and "internal table" can be used for forming complex data types and for directly structuring complex data objects.
Self-defined data types can either be inserted locally in the program or for all programs in the ABAP Dictionary.

Local program types
TYPES can be used to create local program data types . User-defined data types can be elementary or complex.
Self-defined data types can be used in the same program at any point where predefined data types can also be used, that is, when creating data objects and when typing formal parameters and field symbols.

Cross-program types
In the ABAP Dictionary, cross-program data types are either structured as data elements, or are created with a complex structure from existing data types.
In addition to the real data types, the ABAP Dictionary also contains the groups known as type groups . A type group is defined as a piece of ABAP code that is maintained in the ABAP Editor.
The statement used to begin a type group is TYPE-POOL . Type groups can contain constant definitions as well as type definitions. The names of all types and constants defined in a type group must have the type group name followed by an underscore as a prefix.
To enable types in a type group to be addressed in a program, the type group must be declared with TYPE-POOLS in the program that is using it.