A tag group 
represents a structural element for grouping tag elements according to your application's needs. For instance, a group "Temperature" and "Liquid Level" can be created in the same device to distinguish between data sources related to temperature and those related to liquid levels.
Additionally a tag group can be used to assign a certain time stamp to all tags included in the tag group. The time stamp can be specified by a DATE_AND_TIME variable in the PLC project.
For further information, please refer to the new tag group wizard and add a new tag group.