Had some debate with myself if I should start by focusing on training for shopping pages (product pages & product reviews) - because that might make some money; or start by training for forums - which I'd enjoy a lot more. Or build a more general system which would definitely never work and never get finished.

Google actually let you filter by "discussions" until a few years ago, so they certainly do this kind of classification. It didn't work perfectly but sometimes did the trick. Don't know why they removed that feature.