Custom taxonomy names can be found in the browser bar on the backend when viewing or editing an item.
Example: edit-tags.php?taxonomy=mycategory&post_type=myposttype
In the example above, we are editing a custom taxonomy called mycategory under a custom post type called myposttype. If I wanted to output a list of categories assigned to a myposttype page, I would use %%ct_mycategory%%. Alternatively, if I wanted to output the description of an individual mycategory on the category archive page, I would use %%ct_desc_mycategory%% .