INITIALIZATION IS  THE FIRST EVENT WHEN  U R WRITING THE 
REPORT.
IT WILL TRIGGERS BEFORE DISPLAYING THE SELECTION SCREEEN.
THIS EVENT IS USED TO PROVIDE DEFAULT VALUES TO THE 
SELECTION SCREEN.
BUT DIRECTLY WE GIVE DEFAULT VALUES TO THE SELECTION SCREEN 
USING DEFAULT KEY WORD FOR SELECT-OPTIONS STATEMENT.

THE USE OF INITIALIZATION EVENTS IS WE HAVE TO CALCUTATE 
THE LOW AND HIGH VALUES IN THE INITIALIZATION EVENT THEN IT 
WILL DISPLAY ON THE SELECTION SCREEN.
FOR EX:
LOW  = SY-DATUM.
HIGH = 10 + SY-DATUM.
HERE WE ARE NOT GIVING DIRECT VALUES TO THE SELECTION SCREEN
WE ARE CALCULATING THE HIGH VALUE THEN AFTER WORDS  WE ARE 
GIVING THE HIGH VALUE TO THE SELECTION SCREEN.
 
SELECT-OPTIONS : EVBELN FOR VBAK.
INITIALIZATION.
EVBELN-LOW = '7580'.
EVBELN-HIGH = '7590'.
EVBELN-SIGN = 'I'.
EVBELN-OPTIONS = 'BT'.
I MEANS INCLUDE. THOSE  7580 AND 7590 ARE INCLUDED.
IF WE USE E EXCLUDE.
BT MEANS BETWEEN .

Preethi is right but will like to mention here.


INITIALIZATION.
EVBELN-LOW = '7580'.
EVBELN-HIGH = '7590'.
EVBELN-SIGN = 'I'.
EVBELN-OPTIONS = 'BT'.
after this you need to write APPEND EVBELN.Otherwise it 
won't work.

Thanks