This standard equation is used to compute the skin depth. An explanation can be found here.
Since data sources for materials properties typically present permeability in term of relative permeability (μr), that is what is used for input in the calculator.
Most typical materials used for signal and power carrying cables - e.g., copper, aluminum - have a relative permeability of 1. Materials with magnetic properties such as steel and iron have relative permeabilities ranging from hundreds to hundreds of thousands.
Here is a table of materials with values.
Numerical inputs are not error trapped.