The supplementary source need not be formal. If you are looking at a python, but the name is not coming to mind, someone may say “Monty” and that may be sufficient to sum with the non-verbal stimulus to evoke the response “python.”
A type of controlling variable with the following features:
It sums with another controlling variable to evoke a response in a case of multiple controlling variables
The supplemental variable, by itself, is never likely to be sufficient to evoke the response
The type of controlling variable