Question: Three argument signum

Does anybody mind clarifying what the three argument signum means?

I have the expression

signum(0,R-sqrt(R^2+z^2),1)

in which z>0, R>0. I guess I know what signum means, however in this case according to the help it is considering signum of 0 and then in the help it is talking about some environment variable. I just dont quite understand...

Does it mean Maple has issues to clarify the sign of R-sqrt(R^2+z^2) ??

Thanks

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