As of Destiny 2, nodes can exist as part of "Exclusive Groups". These differ from exclusive sets in that, within the group, many nodes can be activated. But the act of activating any node in the group will cause "opposing" nodes (nodes in groups that are not allowed to be activated at the same time as this group) to deactivate.
See DestinyTalentExclusiveGroup for more information on the details. This is an identifier for this node's group, if it is part of one.