You use ClockAssist Free/Starter as a memory aid within your existing software to base your time registration on. In that case, it can be useful to mark periods within the ClockAssist popup as booked, so you don't do it twice.

With your mouse, you can select a period on the timeline. If you hold down the CTRL key on your keyboard while releasing it, the selected period will be marked. This way, you know exactly what you have already booked.