When you click on the track topic button at the bottom, and a topic goes into your subscriptions, how are you notified if there is a new reply to the topic? Do you get an email? or do you have to go in and look at your subscriptions to see?
Just tried it, it didn't e-mail me or PM me =P So it would be safe to assume, from my point of view, that you DO have to track the topic manually via your User CP.
You can either track a topic, which just puts it in your subscriptions in your preferences or you can click e-mail topic so it e-mails you when there is a new reply.