When you read a topic:
- you see its category via the info-line displaying "posted... in..." (e.g. New features)
- you see its type via the icon for "Idea, Question, Bug..."
When you post a topic:
- the window proposes to "post in"... for the type ;-) (not the category)
- and the window proposes to select the "category"
As you can see "post in" is from one hand used for the category and on the other hand for the type.
I suggest to harmonize all this to help the user clearly differentiate type & category.
A possibility could be to "re-label" the fields in the new topic input formular:
1. "Post in" could be relabeled "Type" (of topic)
2. "Category" could then be relabeled "Post in", as displayed after it's posted (as it's really the destination where the post will be published).