Community Topics which are submitted are organized by Channels. The possible Channels available to the User when they go to submit a Topic on the User Portal are defined by your Admins, but some examples could be:
Your Admins can configure the helpdesk so that some Community Channels can only be submitted by particular Usergroups. For example, you might take Feature Request topics only from members of the Advanced Users Usergroup, or Bug Reports only from Registered Users.
Your helpdesk may also require users to select a category for their Community Topic. The categories available are also created by your Admins. These allow you to categorize the Community Topic and helps to provide more specific context about what the Community Topic relates to.
Examples of some possible sets of categories: