From b2evolution manual

[edit] Basic Use of Categories

The Categories tab is where you assign categories (and sub-categories) to each specific blog on your site. You must create at least one category for each blog you wish to post to. You can duplicate category names in different blogs and they will remain distinct. For example, if you have a category called "Habitat" in your Elephants Blog you will also be able to have a category called "Habitat" in your Hyenas Blog. The two categories will have distinct id's in the database and posts will not be confused between the two blogs.

Categories can be useful on your blog because users can sort posts by category.

[edit] Advanced Use of Categories

It is possible to use one category across multiple blogs. This is called "cross posting". You can set category options like this by editing your conf/_admin.php file. The same parameter allows 4 options:

• only post in one category in one blog
• post in multiple categories in one blog
• post in multiple categories in multiple blogs
• move categories from one blog to a different blog

[edit] How Can I Display the Categories on My Blog in a Specific Order?

Since 3.1.0-beta is possible to manually order the categories.