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>