The parent item can be used to control how many questions are on each page. For example, you may want three questions on the first page, five on the second, and four on the next. This can be achieved using the parent child grouping which is the parent item field on the question.
The parent item allows you to group multiple questions together for the multipage views. Any question that is set to top will appear first on a page, and any other questions with that question as the parent will also appear on the same page.
For example, both these questions will appear on the same page, as the second one has the parent item set to the first question.
Notice how the child items are indented slightly in a similar way to Joomla sub menu items. You can only have one level of parent and child, you can’t have child items of child items. Also the parent and the child item must be in the same category. Each parent item can have as many child questions as you like. The parent child grouping has no impact on the all questions on one page view, it only works for the default multipage view and the save as you go view which is also multiple pages.
It should be noted that when you are using conditional branching (next question id), the last question on the page will determine the next question. Any conditional branching in other questions on the same page will be ignored.
If your next question id refers to a child item, as the parent is skipped, the child will appear on the page by itself. All subsequent child items in that group will appear one question per page, until the next parent question is displayed, which will have its child questions on the page too.
When you add a child question from a different category, it inherits the category from the parent.
If you change the category of the parent question, all child questions will also have their category changed.
There are only two levels, parent and children. You cannot have children of children.
A parent question with children cannot be changed to be a child of a different parent question. You must first remove all the children questions.
In regards to ordering, the first child question will have order of 1, the second child question will have order of 2 etc. This is independent of the ordering of parent items.