PN:DFF on billings tab when editing a lease

 Hi all,

I am trying to active the dff on the billings tab when you need to edit a lease. There is a dff, but I have been unable to find it on the list of availalbe dff's for property manager.

I want a user to set a flag of Y or N when they add another line on the billings tab for editing a lease. i.e. Rent may already have been charged and now user needs to add another line for let's say effluent charges. I Want to be able to track when this happens and therefore I need to force user to set a flag via the dff.