Does anyone knows the relation between the Due Date of the Invoices and the field tccom010.eded (Days after due date) in the customers master?

The "Extra Days after Due Date" field in the Customer Master can be used in calculating a Cash Forecast (in CMG). The intent is to let you add known or expected delays in receiving payments to the Due Dates of the customer's invoices. A negative number means you expect payment before the invoice due date.

There is similar functionality on the supplier side to let you quantify for cash forecasting how you plan to "stretch out" your payments suppliers. Or whether you plan to pay your supplier early. (No need to take a poll on how companies use this one!)

Specifications for the forecast calculation determine whether the cash forecast actually uses "Extra Days after Due Date".