Sample code1:

function zoom_available return Boolean is
form_name  varchar2(30) := name_in(‘system.current_form’);
block_name varchar2(30) := name_in(‘system.cursor_block’);
begin
if (form_name = ‘DEMXXEOR’ and block_name = ‘ORDERS’) then
return TRUE;
else
return FALSE;
end if;
end zoom_available;

function zoom_available return Boolean is
form_name  varchar2(30) := name_in(‘system.current_form’);
block_name varchar2(30) := name_in(‘system.cursor_block’);
begin
if (form_name = ‘APXINWKB’ and block_name = ‘INV_SUM_FOLDER’)
then
return TRUE;
elsif (form_name = ‘APXINWKB’ and block_name = ‘LINE_SUM_FOLDER’)
then
return TRUE;
else
return FALSE;
end if;
end zoom_available;

2] STYLE:
This function returns a integer value. This function allows to override the execution style of Product specific events, but it doesn’t effect generic events like when-new-form-instance. Possible return values are:
1. custom.before
2. custom.after
3. custom.override
4. custom.standard
By default it returns custom.standard.

Sample code:
 
function custom.style(event_name varchar2) return integer is
begin
if event_name = ’MY_CUSTOM_EVENT’ then
return custom.override;
else
return custom.standard;
end if;
end style;

3] EVENT: 
This procedure allows you to execute your code at specific events including:
  –    ZOOM
  –    WHEN-NEW-FORM-INSTANCE
  –    WHEN-NEW-BLOCK-INSTANCE
  –    WHEN-NEW-RECORD-INSTANCE
  –    WHEN-NEW-ITEM-INSTANCE
  –    WHEN-VALIDATE-RECORD
By default this routine must perform ‘null;’

Sample code:
procedure event(event_name varchar2) is
form_name varchar2(30) := name_in(’system.current_form’);
block_name varchar2(30) := name_in(’system.cursor_block’);
begin
if (form_name = ‘XXBI’ and block_name = ‘xxcc’) Then
if(event_name = ‘WHEN-NEW-FORM-INSTNACE’)
–Write your code here
elsif(event_name = ‘WHEN-VALIDATE-RECORD’)THEN
–Write your code here
else
null
end if;
end if;
end;

How to make the changes get affected?

Once you make all the necessary changes, compile the pll and generate the PLX file. Since the CUSTOM library is loaded once for a given session, a user must log out of the application and sign-on again before any changes will become apparent.
Forms Personalization: an alternative of custom.pll
In older versions, prior to 11i, Custom.PLL was most prominently used for adding additional features in the seeded form but the latest version of Oracle EBS comes with the feature called as Forms Personalization which allows even an end user to alter the seeded forms functionality using an user interface called the Personalization form.
Advantages of Forms Personalization over Custom.PLL:

• Forms personalization can be used by an user with limited PL/SQL knowledge. 
• Changes take place immediately on reopening the form.
• Anything which can be done using Custom.PLL can be done using Forms Personalization also.
• Personalizations are stored in base tables related to Form Personalization.
• CUSTOM.pll is a single file/entity, hence only one developer can make changes to CUSTOM.pll at any given point in time. This is not a restriction in Forms personalization.
• Easy to disable/enable with click of a button.
• Can be moved easily through FNDLOAD from one instance to other.
• Can be restricted at site/responsibility/user level.
• Personalization stores who columns with which we have the ability to track who created/modified it where as in CUSTOM.PLL we don’t have that ability.