Thanks to Taxonomy_Single_Term, a library created by WebDevStudios, it made my work much easier on creating this plugin. With Categories Metabox Enhanced, you can: Change the built-in Categories metabox to a single term UI, which means replacing the checkboxes with radio buttons or a select drop-down. Apply the single term UI to other hierarchical taxonomies in the plugin’s Settings page. Customized the single term UI by setting these options: Priority and position of the metabox placement Title of the metabox If child-terms should be indenting If adding of new terms from the metabox is enable