if we write events in sequential,then the events will be
executed seuentially and if not then the drver program will
execute the report in sequential order.if we write in
sequential the performance will increase.

if we write end-of-selection event after star-of-selection 
then it will stop the processing block if we dont wrote end
of selection by defautly it will triggers after start of
selection.

top of page event will trigger after intialization,at
selection screen and before start-of-selection

Not Necessary for the events to be in a sequential order.
During run time, the events will be triggered automatically
in the sequence. 
But it is suggested to write the events in a sequence for
the user to easily understand the flow of program and for
the efficiency purpose.
END-OF-SELECTION: Will be normally used to free the memory
and display the write statements once all the fetching is
done. But it is not mandatory or necessary to write
END-OF-SELECTION. If any event is missing in the sequence,
the next event will be triggered automatically.
INITIALIZATION  initializes the filed values in the
selection screen. This is done before at-selection screen,
before PBO.
TOP-OF-PAGE: This is used to display the required text on
the top of the page as header. This event will come AFTER
INITIALIZATION event only !!
Here is the flow of events:

INITIALIZATION
AT SELECTION-SCREEN
AT SELECTION-SCREEN OUTPUT
START-OF-SELECTION
END-OF-SELECTION