ATX 2.1 is a new standard for the 20+4 connector, where ATX 1.1 is the 20 pin connector. ATX12V standard moved from 1.1 because the migration of CPU voltage regulation from a 5V load to a 12V load. Requirements specific to the 5V CPU regulation have been dropped from the standard as of ATX12V 1.3. SATA was specified, but not required, in ATX12V 1.3.
24 pins are introduced in ATX2.0, also dual rails.
24 pins become the standard in ATX 2.01, +5Vsb is now 2.5A instead of 2.0A.
With ATX2.1, increased loads levels are the norm. With more +12Vdc parts asking for more power, PSUs 12V rails are required to conform to the new power demands. If the demand is larger that 18A, a second +12v rail should be added.