Since an UI layer is in the same design space, it shares dimensions with the board. This makes it easy to draw precisely positioned annotations and marks. As the UI layer data is manipulated by plugins/scripts, it can be real-time and/or interactive.

This video demonstrates the UI layers in pcb-rnd. These layers are virtual drawings rendered over the board. UI layers can help in debugging and can enable feature plugins to draw over the board without changing any board layers.

The clock is rendered by an embedded awk script.

Note: when the board is flipped and viewed from the solder side, the clock seemingly goes backwards. It's because we are looking "out from inside the clock". The desktop video recording software made the jumps in the seconds - in reality the render is smooth and real-time.