There are topics (tags) and then there are communities. You can only put your post in one community (or don't put it in your community and post it directly to your blog)

Topics are another things you can just use different words in the topic field and they'll show up on pages that go and find posts that use those topics.

Your post can only really exist in one community... but PeakD does allow people to create a Cross-Post it's like a duplicate post sort of thing your original post will still be in that original community but we'll help you show it off in the other community. But only works for those using PeakD presently