Hi drak86
It's not the ID but alias of Joomla tag, if you have K2 tag which its name is the same with an alias of a Joomla tag, it will be imported and replace the Joomla tag.
For example, you have a Joomla tag called 'news' and alias is 'news' and a K2 tag called 'news'. When you import, this K2 tag will replace Joomla tag 'news'.
It doesn't relate to ID.