The use of hierarchy as an organizing principle keeps coming up. I think we are attracted to hierarchy because of its neatness, even though in fact the real world is organized more like fuzzy sets. Fuzzy sets are hard to comprehend, nearly impossible to draw, and can't be slotted neatly into an application.
When people talk about FRBR, they are often focussed on the Group 1 entities, and those are seen as hierarchical. They tend to be shown as:
as if we'll fit all of our intellectual works into such a neat hierarchy. T'ain't so. Of all of the relationships that are talked about in FRBR (I almost said "expressed" but that term has now been given a new meaning in this discussion) I think these are the least interesting. And they become even less interesting when we move beyond the traditional inventory control function of the library catalog and begin to see ourselves as navigating in a knowledge universe. But first let me tackle the Group 1 entities.
There are complaints (or remarks, depending on the context) that we don't have an agreed on definition of Work, and that the division between Work and Expression is unclear. They are unclear because in real life there isn't a neat hierarchy that just needs to be modeled. What is a Work is entirely contextual -- when I'm looking for an article, the article is a Work. When I'm subscribing to a journal, the journal is a work. When I'm on iTunes a song is a work, when I'm in the music store the album is a work. A Work is the content I am seeking at that time. In the imaginary universe where I get to create my bibliographic system, a Work will be defined as: anything you wish to talk about, point to, address. So a book-length text is a work, an article in a journal is a work, a journal is a work, a book chapter is a work -- all at the same time and in the same system. For one person the book Wizard of Oz and the movie Wizard of Oz will be a single Work. To a film buff, the director's cut of Blade Runner and the original release are distinct works. To be a Work, it just has to be definable and have a way to name it, that is it has to have an identifier. But anything can be a Work. As a matter of fact, I probably won't use the term Work at all in my universe.
As for Expressions, there will be very obvious Expressions of Works, and there will be fuzzier Expressions. There will be Expressions that express more than one Work. Expression is a relationship, not a subset. If you don't have to organize your bibliographic universe in a hierarchical way, then the need to slot each Expression under a Work goes away, although the relationship can remain.
I'm less sure about Manifestation and Item, even though these are the most concrete of the Group 1 entities. Are they a legitimate focus of a Knowledge Management system, or are they about managing physical objects? When I think about some of the uses of bibliographic data, for instance as citations in a text publication, Manifestation seems to be mainly about locating -- so if I've quoted a passage from a book, I need to cite the manifestation and the page because that's the only way that someone else can find that exact quote. When I include a URL in a document that links to a particular digital manifestation, I am giving the user a direct link to the location. Manifestations and Items will be of interest in some instances, say to rare book collectors, but I'm not at all sure that those instances justify the emphasis they have been given. And if the purpose is primarily inventory control, then I think those relationships will be managed to the extent that they matter to the library. For example, a public library may not terribly care which manifestation of the book Moby Dick is on its shelves, although its inventory system will need to know the barcode, and its acquisitions system will need to store how much the library paid for it and the provider.
The truly interesting relationships in FRBR are those between and among these entities, and those are ones that I have not seen explored. These are the relationships between things: thing1 is a translation of thing2; thing3 is an abridgment of thing4; thing5 extends thing6 in this certain way; thing7 cites thing1; thing8 continues thing3. This is where we get real value, where we provide various interesting paths through which seekers can navigate. This is what we don't provide explicitly in our catalogs today, although a human user may be able to intuit some of these relationships among the works we present.
We have so narrowly defined bibliographic control in libraries that it doesn't really include the relationships between intellectual products, except to the degree that we might make a note that one thing is a translation of another thing. But we see those relationships as "extra" or "secondary," and yet they are the very essence of knowledge creation. It astonishes me that we have focused so completely on the physical items that we have essentially missed what would make our catalogs intelligent.