Groups are identified by a double cross ('#') after the opening << characters. While bands <<:.../>> are not nested, groups can be nested and so are embedded into a tag pairs <<#...>> <<#/....>.
Unlike header and footer bands the name of the group tags are not fixed. Practically any name can be used if it does not contain spaces. However the opening token must match the closing token. For practical reasons the names should match the database which is referenced inside of the group.
<<#CUSTOMERS>> comment: we list all customers in this group
<<#ORDERS>> comment: we list all items ordered by the current customer
Note: The name of a group will be passed to the event OnReportState while creating a report. This event has to decide if a group should be processed (again) or not. It can create a sql query and calculate sub totals.