Custom CPT Taxonomy
WordPress allows the creation of custom taxonomies. You've seen categories and tags for posts, these are the taxonomies. A lot of plugins that create Custom Post Types will have custom taxonomies. For example, Woocommerce uses product_cat for its categories and product_tag for tagging.
The Events Manager plugin has a CPT of event, a custom category taxonomy of event-categories, a custom tag taxonomy of event-tags. Here's an example of how you would set that up in Reactor:
To get access to the custom taxonomy options click the show advanced options link under the Custom Post Type setting.
Find Custom Taxonomies
The CPT will generally have a menu item for its taxonomies like posts do. Though they may be named differently. The Events plugin mentioned above has a menu item "Event Categories". Click the category menu item and then get the taxonomy from the url parameter.