FIRST TOPIC-TAG IS SPECIAL
What: The first topic-tag is considered a category and can never be changed.
Why: First topic-tag helps other users to quickly look at your post and understand what your post is going to be about (along with your thumbnail and title).
Suggestion: Try to make this topic-tag relate to the most important aspect of your post. Don't use overly generic tags for this first one.

I always wondered about this. What about when people post in the wrong community. Shouldn't there be a feature where others can let you know you've posted in the wrong community and either suggest the correct community or have it go to your blog. It would help organize content.