In my real life I am facing lots of problem without having knowledge of these concepts. Right now I am sharing these concept with you.Hope it'll help you a lot. When we define a variable or an internal table in a program after executing it processor allocate memory for that variable. Example: data: var_data type c. write : 'Memory allocation test.' Here processor allocate memory for that variable.When we write: Clear: Clear Var_data. Its clear the allocate memory for that variable var_data. Free: Free Var_data: The memory allocate free for that variable and can be used for others variable. Refresh internal_tab: Refresh is used only for internal table.when we used refresh internal_table its clear all the entries from the internal table, but memory allocation is still in exist and we can use in within the program. Delete Var_data or itab_internaltable: Its delete the variable value or internal table value based on condition.