It is in the file Precision.lxx
Excerpt from the same file...
//=======================================================================
//function : Confusion
//purpose :
//=======================================================================
inline Standard_Real Precision::Confusion()
{
#ifdef PRECISION_OBSOLETE
static const Standard_Real Precision_Confusion = 1.e-7;
return Precision_Confusion;
#else
return 1.e-7;
#endif
}