Personal notes is a new option with which you can note your opinion about a particular player by creating a note in their profile.
In it, you can describe your impressions of their behavior in the game and, upon the next encounter as a partner or opponent. You can find the "Personal notes" button in the general information about the profile of a given user, next to the button for his statistics.
IMPORTANT: Only players with active VIP status can leave notes on the profile of another player!