From The Railway Encyclopaedia (EF Carter, Harold Starke Ltd 1963):

Aspect - The indication given by a signal to the driver of an oncoming train.

Three Lens Unit

Three Lens Unit (left) can display three aspects - green (upper), yellow (centre) or red (lower). On some units the yellow or green lights can flash, increasing the number of aspects.

The 1965 and 1988 Rule Books use the term multi-aspect as a synonym for multi-lens units - but note that searchlight units (right) are also multi-aspect.

Searchlight Unit  (Right) has only one lens but is three-aspect, displaying green, yellow or red.

The 1965 and 1988 Rule Books imply (incorrectly IMHO) that searchlight signals are not multi-aspect.

Searchlight Unit

Two Units. The number of aspects can be increased by stacking units vertically. Two units give a total of nine available aspects with non-flashing lights. The New Zealand system uses six of the available non-flashing aspects.

Three Units. Some signals have a third short-range unit that can display a yellow light. It is classed as a Low Speed light.

Aspect

Indication

STOP

Illegal

Caution Low Speed

Proceed at low speed, prepared to find track occupied, and ready to stop clear of any obstruction. Points are in the proper position but track may be occupied or suitable for low speed only.

Illegal

Caution Normal Speed

Proceed at normal speed, prepared to stop at next signal.

Section is clear but signal in advance is at "stop" or is displaying a Low-speed indication.

Advance Caution Normal Speed

Proceed at nomal speed. Section is clear and signal in advance is at "Caution" for Normal speed. Introduced after issue of 1988 Rule Book./p

Caution Normal Speed

Proceed at normal speed, prepared to reduce to medium speed at next signal. Section is clear and signal inadvance is at "Caution" or "Clear" for medium speed only.

Caution Normal Speed

Proceed at normal speed, prepared to reduce to intermediate speed at next signal. Section is clear and signal in advance is at "Caution" or "Clear" for intermediate speed only.

Illegal

Illegal

Illegal

Illegal

Clear Normal Speed

Proceed at Normal speed. Section is clear and signal next in advance is at "Caution" or "Clear" for Normal speed.

Clear Medium Speed

Proceed at Medium speed. Section is clear but suitable for medium speed only, and signal next in advance is at "Caution" or "Clear" for medium, Intermediate or normal speed.

Clear Intermediate Speed

Proceed at intermediate speed. Section is clear but suitable for intermediate speed only, and signal next in advance is at "Caution" or "Clear" for medium, Intermediate or normal speed.

Caution, Medium Speed.

Proceed at medium speed, prepared to stop at next signal. Section is clear but suitable for medium speed only,and signal in advance is at "Stop" or is displaying a Low-speed indication.

Caution Intermediate Speed

Proceed at intermediate speed, prepared to stop at next signal. Section is clear but suitable for intermediate speed only, and signal in advance is at "Stop" or is displaying a Low-speed indication.