Diff betn FIELD <f> MODULE <m> ON INPUT and ON-Request.

Check out this SAP Help, I found :

You can ensure that a PAI module is only called when a 
certain condition applies by using the following 
statement:

<b>FIELD <f> MODULE <mod> ON INPUT|REQUEST|*-INPUT.</b>

The additions have the following effects:

<b>ON INPUT</b>
The ABAP module is called only if the field contains a 
value other than its initial value. This initial value is 
determined by the data type of the field: Space for 
character fields, zero for numeric fields. Even if the 
user enters the initial value of the screen as the 
initial value, the module is not called. (ON REQUEST, on 
the other hand, does trigger the call in this case.)


<b>ON REQUEST</b>
The module <mod> is only called if the user has entered 
something in the field. This includes cases when the user 
overwrites an existing value with the same value, or 
explicitly enters the initial value. 


In general, the <b>ON REQUEST</b> condition is triggered 
through any form of "manual input". As well as user 
input, the following additional methods of entering 
values also call the module:

The element attribute PARAMETER-ID (SPA/GPA parameters). 
The element attribute HOLD DATA 
CALL TRANSACTION ... USING 
Automatic settings of particular global fields 
ON *-INPUT

The ABAP module is called if the user has entered a "*" 
in the first character of the field, and the field has 
the attribute *-entry in the Screen Painter. When the 
input field is passed to the program, the * is removed. * 
behaves like an initial field in the ON INPUT condition.

Regards,

Subramanian V.