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A topic is intended for a stream of changing data while a parameter is something you set at the begin and don't change that often.

But you probably mean the difference between a publisher and a topic? A topic is a channel on which a publisher publishes data. It is a kind of whiteboard (the topic) on which a person (a publisher) writes things, but multiple people (publishers) can write on the whiteboard. It is also possible that no one is writing on the whiteboard (no publishers) but there are people (subscribers) watching it.