ATmega256RFR2 Xplained Pro has preprogrammed data in the ATmega256RFR2's user signature data page 1 with address 0x0100. User signature data is isolated from the main flash and will not be cleared by a chip erase command. Special commands are available to read, write and erase the user signature data pages through a JTAG interface and from firmware. Refer to the ATmega256RFR2's datasheet for details on how to read the user signature data pages.
The preprogrammed data in the user signature data page 1 is documented in Persistent Memory, ATmega256RFR2 Xplained Pro does not feature an XTAL calibration value.