The Enforce Site List feature only allows RSUs to be setup from a Site List, by disabling the Setup and Unload buttons when there is no Site List open. Enforcing the usage of a Site List ensures that a given site exists in a Site List, and that a RSU was setup adhering to that information.
When the Enforce Site List feature is enabled, a Site List must be open to make the Setup button available. When unloading data from a RSU, MCSetup will attempt to match the RSU's header to a site in the currently active Site List. Unloading of the data will only continue if the site is found.
A further option exists to prevent accidental editing of Site Lists. In order to change a Site List this option must be toggled.
Both options can be found in MCSetup's Preferences.