http://i59.tinypic.com/140vlu8.png

Can someone explain to me why are there 3 firmwares on BM site for S7:

600M 4.86TH/S
575M 4.66TH/S
550M 4.45TH/S

Can I use any of these firmwares on any batches of S7?

Example:
I have B2 unit can I put 600M firmware on it?
or I need to use 575M firmware?
what about 550M which batch is that one