The explanation (from https://en.wikipedia.org/wiki/Diode_logic) states that
R is connected to +12 volts to provide the forward bias current for the diodes and current for output drive. If all inputs A AND B AND C are a positive voltage (+6 volts here), current flowing through R will pull the output positive till the diodes clamp the output to +6 volts, the logical 1 output level. If any input switches to 0 volts (logical 0 level), current flowing through the diode will pull the output voltage down to 0 volts. The other diodes would be reverse biased and conduct no current.
Why the explanation says that R is connected to 12 V? Is the 12V obligatory given "1" is 6V? What are the Kirchhoff equations for this... well it is not a circuit. Is this a kind of star topology connection? What is the mathematical nature/description of this? What exactly does clamp mean - does this mean diodes connected in parallel with cathodes to input, anodes to output will sink all the voltage until it is equal to minimum of their input?