When creating Smart Form, what is the usage of (Category Assignments for Catalog Browsing)

Using Smart Forms, you can restrict the list of categories that the preparer can use when using the smart form in SSP by using this field (Restrict to Browsing Category) In Smart form creation page.

My question, there is a section called (Category Assignments for Catalog Browsing) in the same page of Smart Form creation, when I add any category in this section, I can't see any impact on the smart form when creation requisition, so what is the usage of it?