Custom Data 
Custom data is a specific customer data that you enter in Custom Data Maintenance when defining product configuration data (see Custom Data Maintenance). Data fields created in this activity are located on the Custom Data tab page of the respective maintenance activity. Custom data can be set at the global site, then passed to each site created off that installation.
Note
SFC-related data which will be collected in SFC Data Entry during production are defined in Data Field Definition Maintenance and Data Field Assignment Maintenance under SFC category (see Data Field Definition Maintenance and Data Field Assignment Maintenance).
The ability to define customer specific data allows the application to be extended without customization. These custom fields are key to support integration to SAP ERP or other external systems.