iExpense - Package name please

Hello !

i have a question about writing custom validation on the header level while submitting an expense claim. We want to write some custom validation which prevents users from selecting the incorrect template if they chose a particular responsibility. What package should I be looking at to write the custom code ?

I'm aware of the AP_WEB_CUST_DFLEX_PKG package, but this is only for the line level.

I want to write validation at this level:

Thanks in advance