2.5.1 User Notes System
From Joomla! Documentation
Joomla CMS version 2.5.1 User notes system.
This service allows you to create a one to many relationship with notes about each user in your site.
Each user of your site can have notes created and related to the user table.
Accessing the user manager will give you access to this system.
To access the notes feature look for the document icon next to the user name in the user manager user list. Or access the system from the sub menu after accessing the user manager menu.