It gives you 90% coverage in general. That's not 100% for any given domain. You're likely always going to need to know something beyond that common core for any particular domain. However, that 10% is going to be specific to the domain, and not have much overlap with other domains. The 10% that will be helpful for you at work is likely going to be different from the 10% the tI need for work. And you can get a long way on knowing 90% of something, and have the skills to be able to ask questions about the remaining areas of ambiguity.