What are the possible values for columnName when requesting a report through the API

Question

I'm trying to programatically request an Executive Report (on Campaigns) that my users usally run manually.

Following the instructions found at https://developers.google.com/doubleclick-search/v2/reference/reports/request , I can use the request_report() method succesfully if I only selected the Campaign and Clicks columns in the report:

  request = service.reports().request(
      body=
      {
        "reportScope": {
            "agencyId": "xxx", 
            "advertiserId": "xxx" 
            },
        "reportType": "campaign",
        "columns": [
            { "columnName": "Campaign" },
            { "columnName": "Clicks" }
          ],
          "timeRange" : {
            "startDate" : "2016-07-24",
            "endDate" : "2016-07-30"
          },
          #"filters": [ ],
          "downloadFormat": "csv",
          "maxRowsPerFile": 6000000,
          "statisticsCurrency": "agency",
          "verifySingleTimeZone": "false",
          "includeRemovedEntities": "false"
        }
  )

but if I try to add Engine and/or Trans as columns to the report, like this:

    "columns": [
        { "columnName": "Campaign" },
        { "columnName": "Engine" },
        { "columnName": "Trans" },
        { "columnName": "Clicks" }
      ],

I get the following error back:

googleapiclient.errors.HttpError: <HttpError 400 when requesting 
https://www.googleapis.com/doubleclicksearch/v2/reports?alt=json returned 
"columns[1]: Column identifier is missing or have unrecognized values.  Must 
include at least one of: columnName, customDimensionName, customMetricName, 
savedColumnName">

I guess this means that Engine and Trans must have some other name, but I can't find any documentation for it.

Any suggestions on where I can look it up? Thank you