One of the key things to know about the groups is that they aren't classes in the data modeling sense. Why they are therefore grouped at all is not clear, except perhaps it was a convenient way to speak of them. The IFLA FRBR Review Group maintains that the groups do not represent classes and that the ten (or eleven, with family) entities represent the highest organizational level recognized by FRBR. Unfortunately the treatment of them as groups throughout the document tends to contradict this statement. This just adds to the confusion about the meaning and nature of the groups.
Group 2I'm going to take Group 2 first because it is the simplest. Group 2 is a group of "agents" or "actors" that perform actions in the bibliographic environment. The original entities of the group were person and corporate body, but family has been added through the work that was done on the Functional Requirements for Authority Data (FRAD). In most kinds of data modeling the members of Group 2 would be members of a class, and the class would have certain characteristics that define the kinds of things that could be members and their shared characteristics. For example, one could say that all members are people or groups of people, that they generally have names, they perform certain actions, etc. These characteristics would therefore not need to be defined separately for each member of the class, and definitions of members would only include characteristics unique to that class. Because no classes exist in FRBR, each Group 2 entity is described separately through its own attributes, although there is a fair amount of overlap between them.
Note that each of the Group 2 entities stands alone with no dependencies on any other entities. (This matters when we get to Group 1).
Group 3Group 3 is an odd grouping because it has a rather miscellaneous nature. The entities that are described as Group 3 are ones that are needed for subject description in bibliographic data: subject; object; place; event. Not much is said about them because FRBR, not unlike cataloging rules (AACR, ISBD), does not really address subject assignment. It isn't clear to me where these particular entities come from because they are not equal to the "facets" of LCSH (form, chronological and geographic). It would be interesting to know how this particular set came about, since the FRBR Study Group was looking beyond North American practice.
What makes Group 3 odd, though, is not its composition but that it is only a partial listing of the subject elements; the full set also includes all of the members of groups 1 and 2. So the actual meaning of group 3 is: all of the subject entities that are not in other groups.
It remains to be seen what will happen to these entities when FRBR and the Functional Requirements for Subject Authority Data (FRSAD) are merged. FRSAD takes a 30,000 foot view of subjects, and essentially concludes that if you can call it a subject and give it an identity and a name, it's a subject. This aspect of the original FRBR study, which was specifically directed at the charge of defining the elements of a core bibliographic record, could change as the model becomes more generalized, which seems to be what is happening.