Search around here, or Google, you'll find your answers.
I think common sense prevails here...ask your question another way with similar jobs. For example, what does a transmission mechanic do? Are they not equal to an engine mechanic?
The answers basically have to do with specialization. I wouldn't like a "normal" functional tester to test a database, but I wouldn't want a database tester to do functional testing either. Both can have the skills of the other, but they have to have the skills.