Access an operator page by clicking on its marker from the map, or on its name from the list of your connections.
Select one or more tags from the list or add new ones by clicking on the pencil icon.
From the map, you can filter operators based on a specific tag, through the list at the top right.
To add a note instead, click on the + Private note button.
Both the tags and the notes created on the operators will be visible only to you.