It doesn't matter what your freelancing specialism is, jobs from clients can still be broken down into types. Freelance Switch has handily drawn up a list of 20 freelance jobs and how to identify them.
I've run into pretty much all these over the last few years. That said, type 18 I always refuse. I've been offered a fair number of dodgy jobs since I went freelance, a good number of them being asked to write for plagiarism sites.
I only once did a 2 - never again. You learn the hard way how to spot them in future. Likewise the 3.
My favourite clients are a 6. The jobs may not always be interesting but they pay well and that's important when you need regular income.
I'm still waiting for a 1 and a 19...