The widget:type directive in the fields.ini file specifies the type of widget that should be used to edit a particular field in HTML forms. Xataface uses HTML_QuickForm for its form generation so theoretically any widget supported by HTML_QuickForm should work with Xataface.
An autocomplete. text field. This is not to be confused with the yui_autocomplete? widget. The main difference is that this widget does not provide a drop-down list of possibilities, it just tries to complete what the user is typing inside the text field based on a valuelist.
A field that allows users to look-up a record from another table. THis is a good alternative to a select list. It doesn't use a vocabulary, so this is appropriate when vocabularies would be unpractical (for large vocabularies). You do, however need to specify the widget:table directive for the field so that the lookup knows from which table to load the records.