GeomConvert_ApproxSurface Constructor and Precision Code

Hi
I need to use GeomConvert_ApproxSurface.

Here is the man entry for the constructor:

GeomConvert_ApproxSurface
(const Handle(Geom_Surface)& Surf,
const Standard_Real Tol3d,
const GeomAbs_Shape UContinuity,
const GeomAbs_Shape VContinuity,
const Standard_Integer MaxDegU,
const Standard_Integer MaxDegV,
const Standard_Integer MaxSegments,
const Standard_Integer PrecisCode);

I don't understand how what is the PrecisionCode.
The man says:

the index of precision PrecisCode.

But i can't figure what does it means.

Thank you to all for the help.