This property is accessible only for the variables of the object: PmaData and PmaTrendGroup.
Although the property is read-only it is possible to enter initial value into it by script (if it si empty). After that it is not possible to modify the value (until the application is restarted).
The value entered here is used as initialization value for the trend viewer (for the PmgTrendViewer object). The value is entered in the following property: tvTrend.Unit.
var oData = pMe.Pm("/Data");
var sUnit = oData.Item(2).Unit;