The notes tab is used for a variety of purposes, customized to the needs of each organization. Each organization can create their own note types. Common notes types are Financial notes, which can be used to enter messages when a client has an overdue balance. Note types can appear in the Client Module or the Appointments Module. They appear with a colored box as an icon to draw the eye to the note when one has been created for the client.
For more information on Client Note Types see, Add or Change Client Note Types.