Customers and vendors, as well as banks, that you transfer from your feeder system to the SAP GTS system are stored there in the form of SAP business partners. With the exception of employees, business partners for SAP GTS are always of the "organization" type.
You define the exact function that a business partner performs for your company by assigning a business partner role to it in the system. A number of specific business partner roles are supplied for SAP GTS in delivery Customizing.
For the purposes of document replication, you need to map your feeder system company codes and plants that are relevant for SAP GTS as business partners. You create business partners that represent company codes in the "Foreign Trade Organization" role (SLLFTO). The foreign trade organization maps your company with regard to customs and foreign trade law in SAP GTS. Plants relevant to SAP GTS correspond to business partners in the "Legal Unit" role (SLLSIT).
In Customizing for your SAP GTS system, you have to assign the business partners of the foreign trade organizations to the company codes of the feeder system. According to the same pattern, you have to assign the business partners of the legal units to the plants that they represent in Customizing for the SAP GTS system (Global Trade Services → General Settings → Organizational Structure → Assignment of Organizational Units from Feeder System to Foreign Trade Org. and Global Trade Services → General Settings → Organizational Structure → Assignment of Organizational Units from Feeder System to Legal Unit respectively).
You mirror the assignment of the plants to be mapped in SAP GTS to company codes by linking the corresponding legal units and foreign trade organizations to each other (menu path: Global Trade Services → General Settings → Organizational Structure → Assign Legal Unit to the Foreign Trade Organization).