Substituting signals are used in the following cases:

• impossibility of reflecting any ride-allowance signal,
• so-called 'doubtful signal' appeared on the semaphore (this means reflected signal doesn't match to any pattern),
• no light is on,

Besides, such signal must be given, when:

• the train enters the station or manually controlled block, using the wrong track (because the railtrafic in Poland is right-based, so the situation described means the train uses left-side track)
• the train is to leave the station and no exit-semaphore is concerned with the track on which the train stopped

After passing the substituting signal, the speed is limited to 40 km/h, with regard to the possibility of sudden stop. If the train enters the track provided with automatic block control, and this entering was caused by reflecting the substituting signal, the speed is limited to 20 km/h until passing the semaphore reflecting ride allowance signal (unless the loco-engineer was given an order telling the cancellation of the automatic block control semaphores).

Substituting signals are named 'Sz' ('sygnal zastepczy' in Polish) and numbered according to the internal station scheme.

Substituting signal may be reflected on a standard semaphore (if provided) or it can be posted as a separate semaphore, or even as a dwarf semaphore. If separate, 'Sz' signal is not provided with forewarning signal.

SIGNAL GIVEN BY SUBSTITUTING SEMAPHORES
-	Sz 'SUBSTITUTING SIGNAL' [see description above] Substituting signal can be reflected on a standard semaphore, or on specially set two-cell or one-cell or dwarf semaphore.

MECHANICAL (SHAPE-BASED) SUBSTITUTING SIGNAL
	Sz 'SUBSTITUTING SIGNAL' [see description above] Concerning mechanical semaphores, the nature of reflecting substituting signal requires mounting one-cell light head on a mechanical semaphore's pole. The additional blinking white light is the same during the day and night.