Expandable groups are a special kind of group whose most important feature is that they themselves can be assembled of other expandable groups. Expandable groups names must start with the prefix "#XG_' (For example, #XG_GROUP1).
In addition, expandable groups also support the special values *INCLUDING and *EXCLUDING which allows the exclusion of specific fields from a field list.
They can be used in most places where a list of fields or a group name are allowed. Because an expandable group is replaced by the expanded field list it represents (That's why they are called expandable groups), it can be used in field lists which do not allow conventional groups. Refer to the help text of the RDML command's parameter to find out if it allows expandable group name(s).