CMB2 is a developer’s toolkit for building metaboxes, custom fields, and forms for WordPress that will blow your mind.
CMB2 contains several field types.
CMB2 Conditionals is a plugin for CMB2 which allows developers to relate fields so one of them could only appear when one other have an specific value or when is not empty.
CMB2 Taxonomy will create metaboxes and forms with custom fields for your taxonomies using the CMB2 API (and yes, it will blow your mind too).
CMB2 Field Type: Select2
Select2 field type for Custom Metaboxes and Fields for WordPress.