I'm using a spreadsheet with a massive amount of VBA code in written by someone else. I'm trying to figure out how parts of it work and there's one function which looks like this:

Function a_corr(a As Variant, apf As Single)

a_corr = ((af - a0) / (apf - a0) * (a - a0)) + a0

End Function

I am trying to find out what values it uses for apf, but I can't find apf defined or used anywhere else within the code. Is it possible that I'm missing something or that I'm looking in the wrong place?