Alternative Item Number

Hello, when an alternative item number is setup in the item master > other tab (Use:  Always)  (DAX 2009), when you enter the item number on a sales order, an info box appears saying that the item will be replaced by the alt number.

 I need to get to the code that displays this box and replaces the saleslines itemid number.  I've been searching the AOT without success.

I want to use this functionality in another area in the system.

Can anyone help?

Thanks in advance.