This function provide in BIOS ver. 0082, you can find it in the bottom of DRAM Timing Control page.
if FGR mode auto or enabled, BIOS tRFC will unavailable and only can available set tRFC2 value, when it disabled, BIOS tRFC2 will unavailable to set, only tRFC available.
there is some different between FGR mode enabled and disabled, you can choose what mode you want to use.
in general, FGR mode = enabled, more flexible and compatible to use, when FGR mode = disabled it might cause system unstable.