How to nest a field collection? How to nest Field Collections in Drupal 8? 

I have Field Collection installed and working fine. What I want to achieve is that for example, I'll have a box on the front end which may contain 6 tabs of content, or two boxes each containing 3 tabs.

First part is working: I have a 'tabs' Field Collection with which I can 'add another item' (tab).

For the second option I need a 'box' Field Collection that contains nothing but the 'tabs' Field Collection.
Using the Field collection module in Drupal 8 seems to NOT be a good idea anymore. It currently only has an alfa1 release from April 2016). Instead you may want to try to use Paragraphs instead, as suggested also on the Field collection project page, i.e.:


  Paragraphs is likely to replace field collection for Drupal 8. Field collection is on its way to being deprecated. It is recommended to use paragraphs instead of field collection for Drupal 8 projects.