Clients:Notes Tab
From WHMCS Documentation
The Notes tab allows you to enter private notes about a client. The tab's text includes the current number of notes.
Viewing Notes
The notes in the Notes tab are visible to all admins but are not visible to the client. You can check Make Sticky to display the note throughout the client's account and on any tickets they submit in the Admin Area.
This tab does not display the separate notes that you can view in the Products/Services, Domains, and Profile tabs.