Virtual cell in DataBoundGrid

What is the proper way to setup a virtual cell in a DataBoundGrid? What should I set the MappingField to in GridBoundColumn?

When you say virtual cell, do you mean unbound column? If so, you can use any value you want for the MappingName except the column names for the bound columns from the datatable. You use the grid.Model.QueryCellInfo event to provide the values for the unbound column. You can see a sample in Syncfusion\Essential Suite\Grid\Samples\DataBound\GDBGMultiHeader.