Customer extension- extended basic types are still supported after a release upgrade. Successors to basic types from previous SAP releases are automatically combined with the existing extensions. No manual maintenance is therefore required.
SAP development - new IDoc types created by SAP are supplied as new basic types. Manual maintenance on the part of the customer is not required after an upgrade.
Customer development (modification) - customer-defined basic types and any related extensions are not affected by an upgrade. These customer developments, therefore, may require subsequent maintenance.