Does anyone know how I can use a custom setting list in a validation rule? We have a custom setting list that contains State and Country names and abbreviations. I would like to set up a validation rule that prevents a user from entering a State or country not in the list. Is this possible, if so what is the syntax?
The answer is NO - only hierarchical settings.
However, there is VLOOKUP formula function that can reference a custom object where your state and country names can exist. See the zipcode example in the doc for a template.