The "bound" column is the column from the combo or textbox's RowSource that
will be stored in the RecordSource field referenced by the control's
ControlSource.
Typically, the first column of the RowSource is the one that will be bound
to the ControlSource. For example, a RowSource query selects CustomerID and
CustomerName from tblCustomer. Column 1 (CustomerID) will be the bound
column and usually its width will be set to 0 to hide it. That will allow
the combo to display the CustomerName when the dropdown is closed.