public interface ComboBoxItemCriteria
optionDataSource     
 to retrieve what is shown in the drop-down pickList,          and also
 to retrieve individual display values via the fetchMissingValues     
 behaviorSearchForm,
 such as in the          FilterEditor of a ListGrid, or in a          separate SearchForm (perhaps configured via
 ListGrid.searchForm) Explicitly selected option:
 If a user explicitly selects
 an option from the ComboBox drop down pickList,  default behavior is to generate an advanced criterion
 with operator:"equals" to exactly match the
 selected option. 
This reflects user expectation. If you are, for example, filtering a grid of Orders, and you have a ComboBoxItem as part of a search interface that allows you to pick a specific salesRep, then you expect to see only orders handled by that salesRep.
 Exceptions:
 If the target DataSource does not  support advanced criteria, or
 ComboBoxItem.generateExactMatchCriteria has been explicitly set to false, simple criteria will
 be generated. Also note that if a ComboBoxItem.operator has been
 specified, it will be used instead of setting the operator to "equals". 
 Unrecognized value:
 If, instead of picking an option, a user enters some text into
 the ComboBoxItem's  textbox that does not exactly match one of the available options, the
 generated filter criterion will not be an advanced criterion by default.  Instead it will
 behave like a normal TextItem, generating a
 simple criterion value unless advanced criteria were  explicitly requested,
 or the item also has a specified ComboBoxItem.operator. 
This again reflects user expectation. If you are, for example, filtering a grid of Orders, and you have a ComboBoxItem as part of a search interface that allows you to pick Sales Reps, then if you enter just "John", you would expect to see all orders that were handled by anyone named "John".
 Criterion field:
 By default, if a specific value has been chosen,  the
 generated Criterion targets the valueField with an exact match  (with exceptions explained
 above). 
However, if a search value has been entered without picking a specific record, the generated Criterion targets the displayField if it differs from the valueField. This is correct behavior - if we consider the example of showing all Orders entered by any Sales Rep called "John", we'd need to apply criteria to the display field (containing the Sales Reps' names) rather than the valueField (likely to be an ID or similar).
 Developers may override
 this behavior by specifying an explicit  ComboBoxItem.criteriaField.
 If specified this will be used in the generated criteria whether the user selected an option
 from the pickList or entered a new value into the text box.