SGA PGA

could you someone expain more details to me,

I want to adjust SGA and PGA.

1) what is different between following statement, with/without sid='*'

alter system set pga_aggregate_target=3g sid='*' scope=both;

alter system set pga_aggregate_target=3G scope=both;

2) what is different between following statement, scope = both or scope =spfile

alter system set pga_aggregate_target=3G scope=both;

alter system set pga_aggregate_target=3G scope=spfile;