From b2evolution manual

The categories fieldset is located at the top of the right column on both the Simple and Expert post editing screens.

In the first column you must select the main category of the post. There can be only one main category per post.

In the second column you can select additional categories for the post. You can select several such "extra" categories for each post.

Your post will appear in all of the selected categories when browsing your blog by category. However, when the Permalink URL of your post is constructed, only the main category will be used.

[edit] Cross Posting

You can enable more or less cross posting possibilities by setting the $allow_cross_posting variable in your /conf/_admin.php file.

Possible values go like this:

• -1 if you don't want to use categories at all
• 0 if you want users to post to a single category only
• 1 if you want to be able to cross-post among multiple categories (this is the default)
• 2 if you want to be able to cross-post among multiple blogs/categories
• 3 if you want to be able to change main cat among blogs (which will move the posts from one blog to another; use with caution)