How to access a business object for Common Lookup

Summary:

I cannot seem to identify a function or business object that will allow me to use the values of the common lookup to validate the value in the field.

Content (required):

I am new user of this tool. Guidance is appreciated.

My criteria original criteria said use a dropdown from Common Lookups. Approval with limitations simply said a dropdown cannot be used. It did say common lookup cannot be used.

If there is a way to access the business object: common lookups, the criteria below is essentially the same behavior as a drop down. (Just clunkier.)

field: Person.Region of Birth.