Sign(z): What Does It Mean in a Physics Equation?

[pyroknife]

I've seen some equations written with a sign for example the e field due to a uniform disk of charge is sign(z)*2*pi*etc... what does the sign(z) do in that equation? When I see the equation actually used, I just see them ignore the sign.

 

[ohms law]

wouldn't the answer here depend on the frame of reference that you decided to use? I mean, if you decide that coordinate 0,0(,0) is outside of the possible area that is of interest to the problem, then signs wouldn't really matter much, right? On the other hand, if the thing being modeled crosses from a positive coordinate to a negative one (or vice versa)...