How to utilize Cash Positioning and Forecasting Transaction setups

We are trying to map a source attribute from an AP Invoice to a specific bank account so the cash position data is populated within the Cash Position Report. We see the ability to map a source table and column, but no where to map it to a specific bank account so it will be populated in the Cash Position Report. How is this setup to be utilized for this purpose? The documentation is unclear on the steps to do so