Making the Transactions DFF Context sensitive in FAXASSET

My client wants to use the Transactions DFF in the Asset Workbench (FAXASSET) but only needs it populted when changes are made to the Assignments, but not when changes are made in the Books window. The client wants the DFF to be required in the Assignments window and as a result, when someone goes into the Books window, they are requried to enter a value in the DFF here too, even though the DFF values are only relevant to transactions made in the Assignments window. Is it possible to make the Transcations DFF context sensitive so it is not necessary to