Automatic memory managent what if I set the sga_target to a value higher than zero?

This to enable automatic memory management ;I need to understand how it works?

ALTER SYSTEM SET MEMORY_MAX_TARGET=6G SCOPE=SPFILE;

-- Set the dynamic parameters. Assuming Oracle has full control.

ALTER SYSTEM SET MEMORY_TARGET=5G SCOPE=SPFILE;

ALTER SYSTEM SET PGA_AGGREGATE_TARGET=1G SCOPE=SPFILE;

ALTER SYSTEM SET SGA_TARGET=1G SCOPE=SPFILE;

-- Restart instance.

SHUTDOWN IMMEDIATE;

STARTUP;