If you need to have details e.g. explaining what a service includes, so that every time you add this service to a Booking the details are already there, and at the same time have the possibility to modify this text, you can make use of this functionality (available from 6.6 SP2)
First, on an Object Custom Field template create a Custom Field as a text widget and with the label name "Timereport Note". Next, apply this template to all those Extras (Services, Materials and Expenses) where you need to add details that will be displayed in the Extra's Note every time you add them to a Time Report.
See here an example of service "Insurance Equipment" and how to access this note from the Timereport window and from the Edit Project window.