Path Loss Model for POWDER

[praghur]

Hello POWDER community. 

I was reading about the path loss model for the POWDER environment on POWDER use cases. It says that the "The dB received power is not calibrated, so is listed as referred to an unknown reference." This also shows in the plot of received power vs distance as shown in the attached screenshot. 

I don't think I understand what it means about "unknown reference". Could someone elaborate on this or guide me to a document that can help me understand this?

Thanks in advance.