This method is different then the 'getPlaceholderValues' method, this method is used to create a placeholder value array containing dummy values that is used to create a preview certificate. 
Due the fact that this is a class to create default values the placeholder values will be identical to the description 
  167         $previewPlacholderValues = [
   168             'CERTIFICATE_ID' => $this->utilHelper->prepareFormOutput($this->uuid_factory->uuid4AsString()),
   169             "USER_LOGIN" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_login")),
   170             "USER_FULLNAME" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_fullname")),
   171             "USER_FIRSTNAME" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_firstname")),
   172             "USER_LASTNAME" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_lastname")),
   173             "USER_TITLE" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_title")),
   174             "USER_SALUTATION" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_salutation")),
   175             "USER_BIRTHDAY" => $this->utilHelper->prepareFormOutput((trim($this->dateHelper->formatDate(
   178                 $this->unix_ts_format_id
   180             "USER_INSTITUTION" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_institution")),
   181             "USER_DEPARTMENT" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_department")),
   182             "USER_STREET" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_street")),
   183             "USER_CITY" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_city")),
   184             "USER_ZIPCODE" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_zipcode")),
   185             "USER_COUNTRY" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_country")),
   186             "USER_MATRICULATION" => $this->utilHelper->prepareFormOutput($this->language->txt(
"certificate_var_user_matriculation")),
   187             'DATE' => $this->utilHelper->prepareFormOutput((trim($this->dateHelper->formatDate(
   190                 $this->unix_ts_format_id
   192             'DATETIME' => $this->utilHelper->prepareFormOutput((trim($this->dateHelper->formatDateTime(
   195                 $this->unix_ts_format_id
   197             'DATE_COMPLETED' => $this->utilHelper->prepareFormOutput((trim($this->dateHelper->formatDate(
   200                 $this->unix_ts_format_id
   202             'DATETIME_COMPLETED' => $this->utilHelper->prepareFormOutput((trim($this->dateHelper->formatDateTime(
   205                 $this->unix_ts_format_id
   210             $previewPlacholderValues,
   211             $this->userDefinedFieldsPlaceholderValues->getPlaceholderValuesForPreview($userId, 
$objId)