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It is not documented at all but in host reservations one can specify a group by name.
I believe it is a second way (first is inclusion of the host reservation declaration in the scope of the group) to apply a group to a host. If it is the case the corresponding Kea feature is to declare a class without a matching expression with all the parameters (e.g. option-data) of the group and to specify the host reservation belongs to the class, so basically swapping the group keyword for class.