About CUSTOM.pll.

Custom Library (custom.pll) allows to extend/customize Oracle Applications form(Oracle Form) without changing or modifying Oracle Applications code. Examples may include enforcing a newbusiness rule, opening a form using zoom etc. Most of the things that we can do using custom.pll, we can achieve that using Forms Personalization. Since Custom.pll takes the full advantage of PL/SQL so it is having an edge over Forms Personalization for complex customizations.
CUSTOM.pll is used to add extensions to Oracle’s form Functionality. Some of the common scenarios where CUSTOM.pll can be used are
1. Enabling/Disabling the fields
2. Changing the List of Values in a LOV field at runtime.
3. Defaulting values
4. Additional record level validations
5. Navigation to other screens.
6. Enabling Special Menu

Primarily there are two methods of extending Oracle Forms, and these are 
    CUSTOM.pll
    FORMS Personalizations
In this article we will cover the basics of using CUSTOM.pll for extending Oracle Forms

How and why does CUSTOM.pll work?
Every form in Oracle Apps is created using something called as TEMPLATE.fmb. But some of the modules like HRMS have their own HR Specific Templates[HRTEMPLT.fmb]. These template files have form level triggers that make call to CUSTOM.pll. The triggers that can be trapped using CUSTOM.pll in HRMS screen can be different than those which can be trapped for other modules.

Where is this located?
Custom.pll is located in $AU_TOP/resource Directory.
How to add code to this?
Open this pll using the Form builder and make changes to the program units.

How to compile this PLL?
 Once you make changes you need to compile the pll. Use the F60gen to compile it
f60gen module=custom.pll userid=APPS/ output_file=$AU_TOP/resource/custom.plx module_type=library batch=no compile_all=special
While writing code inside custom.pll we should consider following things:
1. We should not run any SQL statement inside this, we can use record group.
2. We should not perform any DML operations, instead we should call database procedure and functions for the same.

For following Events call will go to CUSTOM Library:
 
WHEN–FORM–NAVIGATE
WHEN–NEW–FORM–INSTANCE
WHEN–NEW–BLOCK–INSTANCE
WHEN–NEW–RECORD–INSTANCE
WHEN–NEW–ITEM–INSTANCE
WHEN–VALIDATE–RECORD
SPECIALn (where n is a number between 1 and 45)
ZOOM
EXPORT
KEY–Fn (where n is a number between 1-8)
Custom Library contains Custom Package which is having two Functions and one procedure.
1] ZOOM_AVAILABLE:
This function allows you to specify if zooms exist for the current context. If zooms are available for this block, then return TRUE else return FALSE. This routine is called on a per-block basis within every Applications form from the WHEN-NEW-BLOCK-INSTANCE trigger. Therefore, any code that will enable Zoom must test the current form and block from which the call is being made. By default this routine must return FALSE.

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;

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.