The primary input to most equations is R, a raw number normalized to a nominal full scale of 1. This is typically a binary reading from an A/D converter divided by a full scale value of 2^30, and so is often referred to as a 'voltage ratio', but other input hardware and scale factors may be used.
A few equations for derived parameters use only secondary inputs from other channels; they have no underlying measurement hardware, and so no R input. A good example would be salinity, which is a function of conductivity, temperature, and pressure.