Hi,
- at the start of the internal table (at first)
- at the end of the internal table (at end, at last).
when a field value of internal table changes. In order to format & display the Internal Table data according to the above requirement we use
Control Level Statements , that helps in reducing the lines of code. Also the Internal Table is to be 'Sorted' before using the Control Level Statements.
pls check this link Control Level Statements in ABAP - ABAP Development - SCN Wiki