This refers to people who are not logged into the target Teachme Biz Group.
Specifically, the following are considered "external users" — Viewers of a Manual that has been Released Outside:
・People who do not have an account for the target Teachme Biz Group
・People who have an account for the target Teachme Biz Group but are not logged into the target Teachme Biz Group
Related: Learn about Release Outside
https://help.teachme.jp/hc/en-us/articles/900006443043