First, beware, as AMM still has bugs in 11g. I don't use it:http://www.dba-oracle.com/t_amm_automatic_memory_resize.htm
The sga_max_size sets the hard limit up to which sga_target can dynamically adjust sizes.
Usually, sga_max_size and sga_target will be the same value, but there may be times when you want to have the capability to adjust for peak loads. By setting this parameter higher than sga_target, you allow dynamic adjustment of the sga_target parameter.
Dr Hall has some good notes on the 11gAMM features:http://www.oracle-base.com/articles/11g/Au...ement_11gR1.php