System partitioning enables application-controlled partitioning without having the database controlling the data placement. The database simply provides the ability to break down a table into partitions without knowing what the individual partitions are going to be used for. All aspects of partitioning have to be controlled by the application. For example, an attempt to insert into a system partitioned table without the explicit specification of a partition fails.
System partitioning provides the well-known benefits of partitioning (scalability, availability, and manageability), but the partitioning and actual data placement are controlled by the application.
Oracle Database Data Cartridge Developer's Guide for more information about system partitioning