In normal times, an Entity receives all Stimuli equally. However, it is able by concentrating to focus on particular Stimuli. Their perception by the Entity is then increased, whereas the perception of other Stimuli is decreased. The focus capability depends on the concentration of the Entity. Some skills can improve it too. For example, musician can focus better on sounds because of training.
It is also possible to focus:
Either on all Stimuli coming from a particular Entity,
Either on all Stimuli of a given Medium.
A Medium (or Sense) focus is improved when the number of active Senses is reduced. For example, it is easier to hear something when closing eyes.
The 'focus' command allows to realize this by the following means:
focus <sense> -> the Entity focuses on Stimuli associated to <sense>
focus <entity> -> the Entity focuses on Stimuli coming from <entity>
focus -> the Entity switches back to a normal perception.