R6T3 is the label of a resistor on the mainboard of the Xbox 360, it supplies the necessary voltage for the Xenon (CPU) to burn the so-called eFuses. Burned eFuses prevent older firmwares (any older rev. than 4552) from running on your Xbox 360, thus eliminating the possibility to downgrade to an earlier version of the kernel. The Resistor itself is located in the field T-6. It is a 10k Ohm (10,000 Ohm) SMT resistor (Case 0402) functioning as part of a current limiting circuit.