The value entered here is used as initialization value for trend viewer (for the PmgTrendViewer object). The value is entered in the following property: tvTrend.Unit.
sUnit = pMe.Pm("/data/#vars/Temperature/#ext/trend").Unit;
//or
sUnit = pMe.Pm("/data/#vars/Temperature").Extension("trend").Unit;
//or
sUnit = pMe.Pm("/data").Item("Temperature").Extension("trend").Unit;