From Xojo Documentation
A "hidden" identifier associated with the item identified by the row parameter.
If you want to compare the value of row tag to another value, you should first convert the Variant to it actual data type.
This example populates the RowTagAt identifier with a sequence number.
recID = PopupMenu1.RowTagAt(1) // ComboBox inherits from PopupMenu so it also inherits the RowTagAt method