How to use a DFF in a mapping set for TAD

We are trying to setup our mapping sets, account rules, and transaction account definitions, and are trying to map segment two of our COA to a DFF field on the invoice.

In the Manage Mapping Sets > Create Mapping Set > Input Sources we had our flexfield selected, what do we need to select for the 'Value Set' Dropdown or what is that 'Value Set' dropdown?

In the Chart of Accounts section is where we will select our 2nd segment.