This type of functionality applies to normal input fields where the user can type in any text they want but you want to suggest a preferred set of standard/accepted values.
The definition can be found here.
The map looks like this:
<field> <fieldname>potentialname</fieldname> <features> <feature> <name>combobox</name> <values> <value>value1</value> <value>value2</value> <value>...</value> <value>valueN</value> </values> </feature> </features> </field>