Using the Preprocess Field's Limit field option

I have a preprocess field that I would like to limit to one content type only (it is generated by an implementation of MYTHEME_preprocess_node only when the node is of type "article"). The field currently shows up in all of my content types, but only generates a value for "article" nodes. I see that the "Limit field" option in the Preprocess Field settings is there to allow me to do this, but I'm not sure how to use it. Can anyone lend a hand?

Thanks in advance!