High frequency diode - RF flow in both directions?

Hi, can i ask:

When a PIN diode is forward biased, can an RF flow in both directions such that S12=S21 ? or is it only one direction, please clarify fo rme thank you.

Yes, it may flow in both directions UNTIL diode is forward biased.
Note that when pin diode is forward biased it acts like dynamic resistor, which resistance is inverted proportional to biasing current.
Pin diode may be shown as parallel junction of capacitor and dynamic resistor, for forward biasing. Because it is in real packing, there are also parasitic inductance and resistance in series with diode and parasitic capacitance in parallel with them all.