It would be nice to have a field on the Billable Time form (and perhaps on the Billable Expense form, as well) for internal notes that are not meant to appear on the invoice. I would use this field to makes notes to help me justify the time or expense in case of question from the customer for a deeper explanation of the work that was accomplished during the time or for a justification of the expense.
Alternatively, adding Custom Field functionality to the Billable Time (+/- Billable Expense) form would accomplish the same thing.
For example, I might bill my customer for an hour of my time for background research. I would want only “Background Research” to appear on the invoice, but I would also want to have a place to record a summary of exactly what research I did, in case somebody asks. Currently, I have to keep a separate spreadsheet for such notes.