The language string NO_POSTS is only used two places in phpbb - forumlist_body.html and ucp_main_subscribed.html.
If a user does not have a dateformat defined, which can occur in several ways, for example if the ACP or UCP Date format field is set to Custom, but no format is provided, and the config table entry is blank, which is the default.
When this occurs, the string "No Posts" is displayed in either viewforum LAST POST column or in the UCP Manage subscriptions display. However, this has nothing to do with how many posts there actually are. It will display that even if there are 100s of posts, if the user has no date format. In fact, even if there are no posts, "No posts" is not appropriate to that column, because it does not list Number of posts, but rather INFORMATION about the last post. If there are no posts, the column Posts will be zero.
It would be much more useful if the NO_POSTS string said:
'If the Posts column is greater than 0, your Date format is undefined'
Lest you think this is too rare and trivial to address, I spent over 4 hours troubleshooting this today.