TableGroupAll Lookups based on AccountCode

Hi,

I am trying to create a form and table similar to the PriceDiscTable. In there, it shows a different lookup when you change the AccountCode. When you pick Group, AccountRelation has a lookup for Groups, with Table it shows a lookup for Accounts, and All is blanked out. How can I recreate this on my own table and form? Where or what is the code used to make AccountRelation show 2 different lookups based on what AccountCode is set to?

Thanks