Just an idea.

At the bottom of page 186.

Comprobar que al fundirse el foco permisivo de una señal mientras la señal presenta ese aspecto, la señal queda en aspecto de parada. Según el caso, en rojo en violeta o en doble rojo. El nuevo aspecto se representa en el enclavamiento.

https://www.ctb.eus/es/system/files/pt_senalizacion_urduliz....

Permissive signals

A British lower-quadrant semaphore stop signal with subsidiary arm below
Although British railway operation is based on the block principle whereby only one train is allowed in a signal section, there are situations when another train must enter the section, and permissive signals are used to control that movement. There are three types of permissive semaphore: calling-on, shunt-ahead, and warning signals. Today, all three look broadly the same; they are shaped like a normal stop signal, though only about two-thirds of the size, and are painted red with a white horizontal band running centrally along them. When "on", they show a small red or white light, and when "off", they display a small green light and an illuminated 'C', 'S' or 'W', depending on their function.

Modern-day permissive signals consist of two white lights at 45°, normally unlit. When lit, with the main aspect showing red, they instruct the driver to proceed but be prepared to stop short of any obstruction. When unlit, the driver obeys the main signal aspect. They can therefore function either as calling-on or shunt-ahead signals, depending on their location (the Warning Arrangement in colour-light areas, uses the main aspect in a similar fashion to approach release junction signalling, in this case it is called a Delayed Yellow ).

https://en.wikipedia.org/wiki/UK_railway_signalling#Permissi...

A distinction must be made between absolute signals, which can display a "Stop" (or "Stop and Stay") indication, and permissive signals, which display a "Stop & Proceed" aspect. Furthermore, a permissive signal may be marked as a Grade Signal where a train does not need to physically stop for a "Stop & Proceed" signal, but only decelerate to a speed slow enough to stop short of any obstructions. Interlocking ('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by a signalman) are usually permissive.

https://en.wikipedia.org/wiki/Railway_signal