What is a Plex data structure?

Question

What is a Plex in terms of data structure? I read this question somewhere

Which of the following abstract data types can be used to 
represent a many-to-many relation?
 (a) Tree.      (b) Plex
 (c) Graph.     (d) Both (b) and (c)

The correct option was (d).
I googled the term but not found anything that explains it in simpler terms.

Answer 1

Here's my understanding from this site:

A Plex data structure, is like a linked list of n-sized arrays. Where each array is called as Bead and each bead can contain either information or a pointer to another bead.

It is a more efficient data structure alternative used to represent an "intricate network of interrelated parts".

Here's an example taken from the same site: