The COMBOFIELD component is a FIELD with the extension that an additional icon is rendered inside. When clicking the icon (or using F4-key or ctrl-space-key) then the server side is invoked (attribute ACTIONLISTENER) and a event of type "BaseActionEventValueHelp" is passed.

The server side application can now react correspondingly by typically opening some popup-dialog in which the user receives some value help - e.g. some list of available values he/she can select from.

The COMBOFIELD looks like a COMBOBOX but is more flexible: the dialog that comes up is completely controlled by application side. And the data to fill the dialog is loaded at this point of time when the user invokes the value help.

There are some standard popup-dialogs (class "IdTextSelection" and others) which are available on server side for typical scenarios. 