code: function(syncOptions) {
      if (syncOptions.pixelEnabled) {
        return [{
          type: 'image',
          url: spec.SYNC_ENDPOINT1
        },
        {
          type: 'image',
          url: spec.SYNC_ENDPOINT2
        },
        {
          type: 'image',
          url: spec.SYNC_ENDPOINT3
        },
        {
          type: 'image',
          url: spec.SYNC_ENDPOINT4
        }];
      }
    }
docs: Register the user sync pixels which should be dropped after the auction.
  
  @param {SyncOptions} syncOptions Which user syncs are allowed?
  @param {ServerResponse[]} serverResponses List of server's responses.
  @return {UserSync[]} The user syncs which should be dropped.