When dragging fields from a data source, you also have access to the following merge functions:
Merge Individual Control - If you drag an individual field from a data source, you have the option to merge this with an existing field on the page. This merge will update the name of the field to match the data source element, and will also adjust the data bindings for the field so that its value will link to the data source. This will not affect the type of the control, or any of its visual characteristics.
Merge Repeating Content - You can drag a repeating element from a data source (identified by an 'R:' prefix) onto an existing repeat control representing a table structure to merge this information. This will update the name of this repeat, and its contained group, and add all the fields within the repeat in the data source along side the table contents on your page. The bindings will also be updated to match the data source.
Merge Repeat to a 'select' type field - You can also merge a repeat element from a data source onto an individual control that provides a set of options to select from (eg select, radio, multiple checkbox). In this case, the target field will be updated to display a dynamic set of options as retrieved from the data source. You will be asked to select which particular entry from the data source should be used as the display value shown to the user, and which provides the data value to use in the XML messages.