It is possible to write into this property only if the PmForm window has not been opened yet.
var oNumber1 = oForm.CreateItem("number", "Nmb1", "Number of packages", "Min:0;Max:33;");
var oNumber2 = oForm.CreateItem("number", "Nmb2", "The size of one package in cm", "Subtype:float;Min:19.8;Max:120.5;");
var sSubtype1 = oNumber1.Subtype; //int
var sSubtype2 = oNumber2.Subtype; //float