Why all the different xB24 firmware

I am very new, I have 3 XBEE 802.15.4 Series 1 modules. I see in the X-CTU program/Firmware Explorer there are a lot Function set, why should I pick one over others? Should I always pick the newest Firmware version, if not why

The additional function sets are for specific Digi hardware “adapters” such as the DIO adapter. Unless you are using a specific adapter never use one of these function sets. In some cases these function sets turn off the UART… You don’t want to go there unless you know exactly why you are doing it. If you have any doubt call the Digi RF support team. And unless specifically advised by Digi Support. I would always use the most recent version as it contains the accumulated bug fixes since introduction. And unless you are using a DIGI adapter always chose the generic function set as in the XBEE 24 802.15.4 variant. Again if you have any doubts, a quick phone call may prevent hours of restoration efforts.