What is the most restrictive aspect of a signal without a number?

The most restrictive aspect of a signal without a number is indeed a Stop Signal. A Stop Signal instructs train crews that they must bring the train to a complete stop, as the signal indicates that there is a condition ahead that requires caution or intervention. This aspect is paramount in ensuring the safety of operations, as it prevents further movement until the necessary conditions are resolved or cleared by the dispatcher or signal operator.

Other signals, such as a Caution Signal, allow for some movement but with safety restrictions, while a Proceed at Limited Speed permits movement at reduced speed under certain conditions. A Clear Signal indicates that the track ahead is clear for normal operations. However, none of these other signals impose the same level of restriction as a Stop Signal, which is the ultimate command to halt any train movements.