Versions 11.2 and greater allow the database administrator to choose between two automatic memory management models: Automatic Memory Management and Shared Memory Management. The memory models are discussed in chapters 14 and 18 of the Concepts for Database Administrators Guide.
Automatic Memory Management manages the SGA and PGA automatically. SGA and PGA memory are redistributed as needed. The amount of memory to be allocated to the SGA and PGA is specified using the MEMORY_TARGET parameter.
Shared Memory Management manages the SGA and PGA separately. The DBA must specify an amount of memory to be allocated to the SGA using the SGA_TARGET parameter. PGA memory is allocated by the PGA_AGGREGATE_TARGET parameter.