Some qualifications are assigned and updated automatically, things like the country you are in, how many approved HITs you have, your approval percentage, etc. For others, every requester is different. I usually don't go and look through the Qualifications tab because I think it's a waste of time. If I see a HIT that I think looks interesting and they require a certain Qual, then I'll apply for it. You can tell the HITs that require tests because instead of "Request Qualification" in the description, it will have "Take Qualification Test"