You can execute different parts of programs conditionally or in loops using the standard keywords IF, CASE, DO, and WHILE.
When controlling the flow of an ABAP program, remember that thestructure of the program is made up of processing blocks that cannot be nested. This section describes how to control the flow of a program within a processing block. The keywords covered here do not allow you to branch outside the processing block in which you use them. You can regard this as internal control of an ABAP program, as opposed to the external control provided by events in the ABAP runtime environment.
To control the internal flow of a processing block, you can use control structures to divide it up into smaller statement blocks according to the principles of structured programming.
Unlike processing blocks, control structures can be nested.Branching Conditionally Loops