Re: newbie question alert!
Due to variations in the LED manufacturing process, individual LEDs will have different light outputs, different color/tint and will have a different forward voltage when the spec'd current passes through them. Therefore the LEDs are arranged into like groups according to three criteria: output, colour(tint) and voltage. Each group will have a range. In your first post, you listed two different "bins", UX1L and WX1T. These are read (U)(X1)(L) and (W)(X1)(T) - (output)(colour/tint)(Voltage at rated current). The links posted above will decipher the letters.
The "binned" Vf is the voltage across the LED when the specific currents Evan9162 has listed above. Confusion arises as Vf is also simply the forward voltage across the LED. The forward voltage varies with the current - in general, as you increase the current, you increase the voltage. This curve is very steep in the LED's operating range (i.e. a small change in voltage represents a big change in current). You have to know which context the term Vf is being used.
Note that If is the forward current and that:
Vf x If = Power consumed by the LED.
Evan9162 has also done some excellent testing that has shown that the "binned" Vf for Luxeons drop over time. This becomes very important when driving the LEDs from something other than a current source.
Paul