Ordinarily produces density output: permi2/sq mi (perkm2/km2). The rounding factor is always 1 (one place after the decimal point).
If one of permi2 and perkm2 is omitted, it will be calculated by conversion of the other (which must be numerical or an error will be displayed). But setting one to auto means it will be calculated automatically from the population (pop) and area (mi2, km2) parameters.
Setting switch to any non-empty value causes the metric value to be displayed first.