When event label and visit custom field PV values are the same then you can set the custom field default value wrt to visit.

Use case: When you collect the 'PMCT' visit the 'Art der Untersuchung' value should display default as 'PMCT'.

  {
      "name" : "visit.extensionDetail.attrsMap.art_der_untersuchung",
      "caption" : "Art der Untersuchung",
      "type" : "radio",
      "optional" : true,
      "options" : [ "Klinische Angaben", "Sektion", "PMCT", "PM Ultraschall" ],
      "defaultValue": "field: visit.eventLabel"
    }

Using the above code when you collect the visit the default value will be set.