You should look at whether or not your post deals specifically with a company, program or location. If it does, then you should post in that company's/location's forum. The more general forums, such as MilesBuzz, are meant as catch-alls for topics that don't specifically deal with a program/company/location.
ex. You want to start a thread announcing a 10,000-mile Air Canada promotion. Although this would have a home in MilesBuzz, it has a better home in Air Canada's own forum. This separation of forums makes it more convenient for people interested in Air Canada issues, rather than having them search through a generic forum to find things related specifically to their program.