To mark time, you could setup a category called
Billable
and another called
Non-Billable.

When you time a session, just click on the category to change it. When you do a report, choose Billable or Non-Billable for the Category under the Filters section.
Of course, your billable time might be more complex than that. You might wish to make categories such as
Office Administration
Client Contact
Project Development
Personal

Then when you do a report, Click “All Categories” in the filter and select your billable categories or select your non billable categories to see a summary of your hours.

You could organize your category menu so that billable and non billable times are grouped together. Or you could name billable categories differently such as adding a star like”*Client contact.” Use the Edit->Categories menu.