code: public String buildSubChoices() {
  
          String elementPath = getParamElementName();
  
          // we have to add a choice element, first check if the element to add itself is part of a choice or not
          boolean choiceType = Boolean.valueOf(getParamChoiceType()).booleanValue();
          I_CmsXmlSchemaType elemType = m_content.getContentDefinition().getSchemaType(elementPath);
          if (!choiceType || (elemType.isChoiceOption() && elemType.isChoiceType())) {
              // this is a choice option or a nested choice type to add, remove last element name from xpath
              elementPath = CmsXmlUtils.removeLastXpathElement(elementPath);
          }
          elementPath = CmsXmlUtils.concatXpath(elementPath, getParamChoiceElement());
          return buildElementChoices(elementPath, choiceType, false).toString();
      }
docs: Returns the available sub choices for a nested choice element.<p>
  
  @return the available sub choices for a nested choice element as JSON array string