If you need to create a country list in K2 as a dropdown list extra filed, this will take a long time to enter countries one by one in the extra field edit form, this tutorial helps you to create such field in minutes
This is a very useful K2 plugin, which enables administrators of websites powered by Joomla and K2 to assign more than one K2 extra fields group to a K2 category.
Install this plugin to save time and complexity, you will not need to refactor your extra fields groups.
The plugin is integrated smoothly with K2 forms for categories and items, no need for extra forms or components.
First, edit the category to assign extra fields groups as required, you will select the main extra fields group as normal, then at the bottom of the category edit form, you can assign more extra fields groups. Then, you will find all the extra fields, belongs to the assigned groups, in the extra fields tab of the item edit form.
Group fields can be displayed in Accordion, Horizontal or Flat layout, this can be configured in the plugin parameters
In addition to the above, the plugin can render mutliselect fields as checkboxes, on both frontend and backend.