dBµA - Unknown current in decibels relative to one microamp / dBµA    (i.e., unknown current flowing through the cable or conductor).

dBµV - Probe output or Reading, in decibel voltage (dBµV)

dBΩ   - Probe factor or transfer impedance in decibels

Ω        -  Probe factor or transfer impedance in linear scale

The equation is based on the Ohms law where I = V/R, but in logarithmic scale, the division converts to subtraction and we get the above formula for calculating dBµA. Probe factor can be given in linear or logarithmic scale, when given a linear value it will be converted to log scale and results will be calculated.