BAPI 'X' indicator

1.what is the use of "call fm update task" ? please give me a example senario in which we use it?

2. what is the use of "starting new task"? please give me a example senario in which we use it?

3. In BAPI why does we pass 'X' indicator? How does the 'X' indicator effect database table when BAPI is of any of the modes U = change

D = delete, I = add