what's the meaning of xsi_tye in python

[liuwe...@gmail.com]

hi AdWords API Team,

what is the meaning of the attr "xsi_type",I can't find any infomation about that in reference ,thanks!

# Construct keyword ad group criterion object.
  keyword1 = {
      'xsi_type': 'BiddableAdGroupCriterion',
      'adGroupId': ad_group_id,
      'criterion': {
          'xsi_type': 'Keyword',
          'matchType': 'BROAD',
          'text': 'mars'
      },
      # These fields are optional.
      'userStatus': 'PAUSED',
      'finalUrls': {
          'urls': ['http://example.com/mars']
      }
  }

  keyword2 = {
      'xsi_type': 'NegativeAdGroupCriterion',
      'adGroupId': ad_group_id,
      'criterion': {
          'xsi_type': 'Keyword',
          'matchType': 'EXACT',
          'text': 'pluto'
      }
  }