- specified fields or subfields, so that all of the flows in a given
- group have the same values for those fields. If a flow does not
- specify a given field, that field's value is treated as 0.
+ fields or subfields specified in the <ref column="groups"/> column,
+ so that all of the flows in a given group have the same values for
+ those fields. If a flow does not specify a given field, that field's
+ value is treated as 0. If <ref column="groups"/> is empty, then all
+ of the flows in the flow table are treated as a single group.