In my experience  both No Charge and Do Not Bill slips are both consistently reported as Nonbillable entries in Timeslips.

Thank you both for responding.  I ran a few sample reports and found that the No Charge slip entry is reported as Nonbillable.

Thank you both for responding.  I ran a few sample reports and found that the No Charge slip entry is reported as Nonbillable.

The difference between the two is that No Charge slips can appear on a bill with NO CHARGE in the Amount column. Do Not Bill slips will never appear on bills. So you can use No Charge when you want the client to know they're getting a break; use Do Not Bill when you don't when them to know (like, tracking something internally). As Nancy mentioned, they'll both appear as unbillable charges.