The group is a set of nodes that can be identified by number. The generator will create only one node of the group for each document.
In other words, nodes of the group has the following relationship: Node1 OR Node2 OR ... NodeN.
By default, each item of the group has same probability: 100%/(number of items in the group).
However, the user allowed specifying the probability of each group's item.
All nodes of the group must be located sequentially at the same structure level. The group number is a positive integer. Empty value or 0 means 'no group'.
Limitation: one hierarchy level can contain one group only.