And now a car analogy ...
If Ford acted like a law firm, they would know they sell automobiles. They would probably know they sell some volume of sedans, SUVs, trucks, coupes, etc. But beyond that, they would not know how many of each product they sold. Under SUVs they would not know how many Explorers versus Escapes versus Expeditions were sold. Oh, and in the SUV category there would be some sedans, trucks and coupes included.
Of course if Ford acted in such a fashion, they MIGHT know they sell automobiles
Law firms know what they sell only at the high level because that is all they have needed to know until recently. Although most firms have some taxonomy of matter types, they are rarely used effectively. For most firms, the work gets a high level categorization based on the billing partner's practice designation. This means transactional work can be tagged as litigation if the billing partner is a litigator. The choice of matter type when it is an option, is too often made by a secretary. These well-intentioned secretaries picked the most convenient type or the one least likely to get anyone's attention. Therefore when someone wants to see "Single Plaintiff Employment" cases, the only way to find such a list is manually - which means it never happens.
This is obviously an opportunity for Knowledge Management (KM) to shine. But I predict the usual challenges for KMers who tackle this problem. First - a firm will appoint a committee to develop a 'comprehensive' list of matter types. The Committee will want to make sure every possible matter type makes it on the list, since Fred's Admiralty practice is just as important as the rest of the firms' commercial litigation practice. The result will be a long list of never-used matter types ... and we're back to where we started.
My advice: Firms need to know what they sell, down to a reasonable product level. Finding that reasonable product level is a task for marketing and leadership and then KM can be the engine to continually support this effort. Once firms know the true volume and margins on each of their product offerings, then they will know where to focus their market efforts and product resources.