GroupType: Group type of this inventory item if it represents a placement group. Is null otherwise. There are two type of placement groups: PLANNING_PLACEMENT_GROUP_TYPE_PACKAGE is a simple group of inventory items that acts as a single pricing point for a group of tags. PLANNING_PLACEMENT_GROUP_TYPE_ROADBLOCK is a group of inventory items that not only acts as a single pricing point, but also assumes that all the tags in it will be served at the same time. A roadblock requires one of its assigned inventory items to be marked as primary.