Hello Lama,
Typically, the reference wavelength in the fiber is used to reference the fiber physical parameter specs. So, if the reference wavelength is 1550nm and you chose constant attenuation, this means that the attenuation for all wavelength is the same and it equals to 0.2dB/km at 1550 nm. Now, if the you choose the attenuation from a file, then you may have the wavelength dependent attenuation. The same applies for all other fiber specs.
I hope this answers your question.
Regards,
Ahmad