code: protected function isFillable($field, $record)
      {
          /** If the field is listed in the fillable fields, it can be filled. */
          if (in_array($field, $fillable = $this->getFillable($record))) {
              return true;
          }
  
          /** If the field is listed in the guarded fields, it cannot be filled. */
          if ($this->isGuarded($field, $record)) {
              return false;
          }
  
          /** Otherwise we can fill if everything is fillable. */
          return empty($fillable);
      }
docs: Is the JSON API field allowed to be filled into the supplied record?
  
  @param $field
  @param $record
  @return bool