Nominal messages are carried by signals called signs.
Nominal messages are relatively instinctive and controlled by internal body processes.
If my stomach keeps rumbling, for example, it may signify to a listener that I am hungry, even if I say I‰m not.
The rumbling sound is a signal that we will call a sign. This is analogous to the bell in Pavlov's famous dog experiment.
You may remember that he rang a bell every time just before the dog was fed. Soon the bell alone signified food and caused the dog to salivate.
(Please, however, don‰t relate this type of sign to the word "sign" used in referring to Sign Language.)