The Counterpart tab is where the details of the supplier, individual or entity that the contract applies to will appear.
When creating the contract if the counterpart is a Unimarket Supplier you can select them from the Supplier field, which automatically populates the primary contact for this supplier into the Counterpart Contacts field at the bottom, however you can edit/delete or add additional contacts if necessary. If the counterpart is not a Unimarket Supplier you can enter their name in the Supplier Name field and add any relevant contact information by clicking the Add Counterpart Contact button.
For more information on contracts see Viewing & Managing Contracts or Creating a New Contract.