The property returns the data type as a number. For the list of data types and numbers assigned to them see the Data types VBScript.
The value of this property can be also obtained in the "PmaRoot" item of the INFO system (when looking into the object with this property).
var oNumber = pMe.Pm("/Number0");
var nDt = oNumber.Dt; //Reading the value
oNumber.Dt = 3; //3=Long Writing the value